The rapid development of new batteries for electrocatalysts needs to meet two requirements of being ecofriendly and providing high energy densities [1,2,3,4,5,6,7]. Recently, zinc-air batteries (ZABs) have become a hot spot for research because of their low-cost, high-energy density, portability, environmental friendliness, and durable charge/discharge cycle effects [8,9,10]. Unlike classic lithium-air battery, catalysts loaded on the positive electrode of ZABs are versatile and the battery is designed and assembled under ambient atmosphere. More importantly, lithium-air batteries usually suffer from fast activity decay because of forming Li₂O [11,12], while ZABs could demonstrate strong durability up to hundreds of charging-discharging cycles and reach an ultra-high theoretical energy density of 1300–1400 Wh kg⁻¹ [13].

Principally, the charging-discharging processes of ZAB are based on electrocatalytic oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). The overall charging-discharging processes are listed follows [13,14,15,16,17]:

In the case of OER, it can be divided into adsorbate evolution mechanism (AEM, which considers the metal site as active site) and lattice oxygen mechanism (LOM, which considers the lattice oxygen in catalysts participate in the OER reaction as well) [18]. The overpotential of OER based on AEM is higher than that of LOM because of the formation of *OOH during LOM, which are subject to the linear relationship between *OH and *OOH [19,20,21,22]. On the other hand, ORR can be classified into 2e⁻ and 4e⁻ transfer processes [13,17,23].

The formation of H₂O₂ and HO₂⁻ needs high overpotentials due to the high energy required for the breakage of the O=O, so the whole ORR process becomes inefficient in two-electron process. Furthermore, these H₂O₂ and HO₂⁻ intermediates can also corrode the catalysts and the carrier, leading to the destruction of catalyst structures and degraded overall performances. Therefore, the reaction process of ORR is closely related to the choice of catalysts, and it is crucial to select a catalyst that boosts the 4e⁻ transfer process [24,25].

Currently, among the catalysts that efficiently drive charging-discharging cycles of ZAB, noble metals and noble metal oxides rank as the benchmarks. Particularly, they possess high conductivities, low overpotentials (RuO₂ for OER, ~1.56 V vs. RHE), and ultra-high half-wave potentials (Pt for ORR, ~0.84 V vs. RHE), so they are often used as an important indicator for various electrocatalysts in ZABs [26,27,28,29]. Nevertheless, their expensive price inhibits widespread application. As a result, low-cost alternatives (such as perovskites) show a promising future to replace noble metals for ZABs.

Generally, perovskite oxides are presented in the structural formula of ABO_3−δ which has shown the advantages of cheap prices, simple synthesis, controllable morphologies, optimal stability, and durability [30,31,32,33,34]. The A-site ions are usually alkaline/rare earth ions with ionic radius r_A > 0.090 nm (like La, Pr, Nd, Ca, Sr, Ba, Ce, etc); and the B-site ions are transition metal ions with ionic radius r_B > 0.051 nm (like Fe, Cr, Co, etc.). In a single lattice, the O element and the larger radius A ion together form a cubic compact stack, and B ions are filled in the octahedral gap [35,36,37]. According to their components, perovskites can be divided into single-component perovskites (ABO_3−δ), A site substituted perovskites (AA′BO_3−δ), B site substituted perovskites (ABB′O_3−δ), and perovskites substituted at both the A and B sites (AA′BB′O_3−δ) [38,39,40]. Besides, A/B-site cation ordered double perovskites are also included. The A-site cation double perovskites A′A′′B₂O_5+δ exhibit layered structures with a stacking sequence of [A′O_δ]-[BO₂]-[A′′O]-[BO₂]-[A′O_δ] along the c axis. All oxygen vacancies are confined to A′O plane because of oxygen migration. B-site cation double perovskite A₂B′B′′O₆ possesses alternately occupied transition metal sites of B′ and B′′ cations. The formation of B′O₆ and B′′O₆ octahedra structures have originated from intervening oxygen bridging the B′ and B′′ atom pair that usually impart unique properties to the materials in catalysis (including ZABs) [41,42]. Usually, hetero-valent metal elements are used in place of A/B site ions that can induce defects or change their valences, which can significantly alter the catalytic activities. Moreover, when transition metals of different valence are doped, electron exchange reactions are also generated, which can improve the conductivities of the materials.

At present, there are already many studies on the simultaneous doping of perovskites at A and B sites, and the corresponding catalytic activities of ORR and OER have been significantly enhanced. Some classic examples of perovskites (like Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ [43] and La_0.6Sr_0.4CoO_3−δ [44]) have already been applied in ZAB. Their ORR and OER activities are comparable to commercial noble materials [45,46,47]. In addition to perovskite materials, there are many other materials that possess high ORR and OER activities and are promising for ZABs. Jiang et al. [48] have prepared Co-N/C+N_G catalyst applied to alkaline ORR with an impressive half-wave potential of 0.91 V (vs. RHE). The power density of ZAB prepared by mixing this catalyst with Pt/C+IrO₂ for air electrode has reached as high as 430 mW cm⁻².

ZAB consists of the following components: gas diffusion layers, catalyst layers, electrolytes, separators, and zinc electrodes. The critical component of ZABs are constant flows of oxygen gas from air electrodes. Different from the Li-air battery, the ZAB’s Zn electrode is not prone to disturbance upon exposure to air [49]. Besides above components, the conventional planar ZABs also include positive/negative current collectors and numbers of holes in the top of the battery that closely contact with air [50,51]. However, there is a high possibility of electrolyte evaporation due to the presence of air contact holes, which leads to a significant limitation in the efficiency of ion flow between the air and zinc electrodes [52,53]. Later, the emergence of flow-type ZABs provide a good solution to above problem. The major architecture of flow-type ZABs is the same as conventional planar batteries, but their electrolyte is designed to flow in a circular form [54,55]. Consequently, the flowing electrolyte can allow for ion flowing between these two electrodes. Further, the flowing electrolyte is also useful for mitigating the problem of dendrite formation at the zinc electrode, thus avoiding battery performance decay.

Very recently, flexible-type batteries have become the mainstream for electrochemical applications [56]. ZAB is a good choice for adapted into flexible battery for their outstanding safety, high energy density, and cheapness. However, to design flexible-type ZABs, solid electrolytes need to be developed to replace traditional electrolytes, and electrodes and battery devices need to be designed with sufficient mechanical strength and bending degree of materials [57,58,59,60]. Therefore, there is still a long way to go before ZAB can be widely used in the market.

In addition, the loss and corrosion of ZAB anode metal zinc has been one of the main reasons for the poor battery life [61,62]. Recent studies have shown several strategies of adding surfactants, carbon materials to the surface of zinc metal, or mixing with other metal materials to make electrodes, which can effectively reduce the loss of anode and elongate the battery life [62]. On the other hand, since ZAB anodes have been built into porous structures, the use of 3D conductive host materials and the method of inhibition can also effectively alleviate above problems [17]. Further, when the battery is charged (OER process), the anode Zn metal will bulge to become Zn dendrite; when the battery is discharged (ORR process), the metal Zn will become ionic state and react with O₂ to form ZnO [63,64,65]. Therefore, the consumption of anode Zn cannot be fully avoided. Meanwhile, conventional ZAB anode undergoes the sluggish kinetic process of ORR, and OER leads to high charge/discharge voltage difference, which greatly limits the energy efficiency and lifetime of the battery [66,67,68,69]. To solve this problem, Zhang et al. [70] designed a Zn-Cu/Ni/air hybrid battery, which was different from traditional ZAB. Herein, the ORR, OER reaction at the cathode of the battery is accompanied by reversible redox reactions of Cu⁺/Cu²⁺ and Ni²⁺/Ni³⁺, which can generate additional high discharge voltage plateaus and low charge voltage plateaus, thus improving the efficiency of the battery.

Morphology control can enhance catalysts’ activities [71,72,73,74]. Common morphologies used for OER/ORR are nanofibers, nanoparticles, hollow structures, core-shell structures, etc. [75,76]. For instance, Kim et al. [77] designed a novel heterojunction oxygen catalyst (PrBa_0.5Sr_0.5)_0.95Co_1.5Fe_0.5O_5+δ (PBSCF)/3D porous N-doped graphene (P-3G). The insertion of 3DNG into PBSCF nanofibers by coaxial electrospinning and the accompanying synergistic effect of this A-site defect of PBSCF greatly enhanced the ORR and OER activities of pristine PBSCF. Due to the introduction of 3DNG, the d-band centers of Co and Fe in the pristine PBSCF are brought close together. This makes P-3G high ORR half-wave potential (0.82 V vs. RHE) and low OER overpotential (1.56 V vs. RHE). The ZAB with a P-3G catalyst coated on the surface of the gas diffusion layer showed a peak power density of 128.5 mW cm⁻² and could operate for more than 50,000 s (110 cycles) at a current density of 10 mA cm⁻².

The Co/Fe oxy (hydroxide) layer is formed due to LOER [22,85]. The A-site ions in Perovskite oxides dissolved within the electrolyte and diffused across the electrode. However, the B-site ions have limited dissolution, allowing for deposition on the oxy (hydroxide) layer. Moreover, the partial dissolution of B-site ions led to a faster deposition on the oxy (hydroxide) layer at the electrode. This could contribute to the stability of this class of perovskite oxygen electrocatalysts. Therefore, although the author did not apply the Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ synthesized by FS method to ZAB, the mechanism and material’s surface kinetics modulation behind it had a profound influence on the search for suitable perovskite oxygen electrocatalysts for application in ZAB charging reaction. Besides, the enhancement of specific surface area and the deeper study of kinetics also provide good reference suggestions for the improvement of perovskite oxide ORR behavior.

For perovskites, the presence of defects in the A, B, and O sites affect the charge distribution, spin transitions, and band structure [87,88,89]. The electrocatalytic activities of perovskite oxygen electrocatalysts can often be improved by tuning the chemical properties of internal and surface defects [78,90,91].

For instance, Jung et al. [92] prepared Ba_0.5Sr_0.5Co_0.2Fe_0.8O_3−δ with a large quantity of oxygen vacancies using a sol-gel method. The results showed that Ba_0.5Sr_0.5Co_0.2Fe_0.8O_3−δ oxygen catalyst exhibited good activities, with an ORR half-wave potential of 0.63 V vs. RHE and an OER overpotential of 1.75 V vs. RHE. Consequently, the author proposed the concept of amorphous layers for performance enhancement, which were formed by heat-treatment at 950 °C for 24 h in argon atmosphere. Due to the presence of Co elements, the amorphous layer on the surface of perovskite thickened when the Co content was high, which led to the decrease of ORR and OER activities. Therefore, the shaping of amorphous layer had a thickness limitation.

Besides oxygen defects, it is theoretically shown that any e_g-filling (σ*-orbital occupation) close to 1 of perovskites have the highest ORR activities [93,94]. The structural/chemical flexibilities and high oxygen nonstoichiometry (δ) can not only maintain the cubic symmetry of crystal structure, but also allow it to be applied as a bifunctional oxide electrocatalyst in the Zinc-air battery [90,95,96,97,98]. Consequently, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ nanofibers were synthesized by electrospinning method [99]. In their work, the author designed A-site defect and oxygen vacancy to enhance the electrochemical performance and applied it in ZAB. The A-site defect introduced more active sites and accelerated charge transport, thus creating a substantial increase in oxygen vacancies. The two synergistic effects further enhanced the Co³⁺/Co²⁺ ratio and increased the O₂²⁻/O⁻ concentration in this oxygen catalyst. Most critically, this work focused on charge redistribution by means of defects, leading to improved electrochemical and ZAB performances. The battery had a power density of 193.1 mW cm⁻², a discharge capacitance of 719.1 mAh g⁻¹ (1.25 V, 10 mA cm⁻²), and nearly no decay for the aqueous ZABs and flexible ZABs after 140 cycles and 100 cycles, respectively.

Guided by the above principle, another Pt@Sr (Co_0.8Fe_0.2)_0.95P_0.05O_3−δ composite catalyst was constructed [2]. XANES and EXAFS results exhibited that the fast electron transfer of Pt-O-Co bonds and the strong electronic interactions between Pt and SCFP induced by the high concentration of surface oxygen vacancies were the key to the performance increase. Moreover, the spillover effect between noble metals and perovskite oxides can significantly reduce the oxygen catalyst surface energy barrier and change the kinetic rate step, thus enhancing performance.

Elemental doping of perovskites can create defective structures, increase the conductivity of catalysts, and even undergo phase transitions [100,101,102,103]. The common dopants are classified into metallic and non-metallic elements, such as transition metals, sulfur, phosphorus, rare earth/alkaline earth metals, etc. [100,101,102,103,104,105,106]. These anion/cation doping can promote the ORR and OER activities significantly [105,106].

Anion-doped perovskite can cause changes in the electronic structure energy and surface properties of the parent perovskite oxide, thus promoting oxygen catalytic activities [105,106,107]. For instance, Gao’s group reported that sulfur doped LaCoO₃ can change the spin state of Co from low spin to intermediate spin and improve the conductivity and substantially enhance the ORR and OER performance [106]. In their work, in addition to the change of Co spin state committed to the enhancement of oxygen catalytic activity, the doping of anionic sulfur also makes the parent perovskite increase the oxygen vacancy concentration, which also contributes to the activity enhancement. Both XPS and XAS results show that the electronic structure energy of Co changes after sulfur doping of LaCoO₃, and the parent perovskite undergoes lattice distortion. Thus, the power density of the ZAB loaded with this catalyst at the electrode reached 92 mW cm⁻², and the aqueous ZABs and flexible ZABs were able to cycle for 100 h and 16 h without degradation, respectively.