Recent Advances in Electrode Design for Rechargeable Zinc–Air Batteries

been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/smsc.202100044. This article is protected by copyright. All rights reserved Recent advances in electrode design for rechargeable zinc-air batteries Jinfa Chang, Guanzhi Wang, Yang Yang Dr. Jinfa Chang, Guanzhi Wang, Prof. Yang Yang NanoScience Technology Center, University of Central Florida, 12424 Research Parkway Suite 423, Orlando, FL 32826, USA. E-mail address: Yang.Yang@ucf.edu Guanzhi Wang, Prof. Yang Yang Department of Materials Science and Engineering, University of Central Florida, Orlando, FL 32826, USA. Prof. Yang Yang Department of Chemistry, Renewable Energy and Chemical Transformation Cluster, University of Central Florida, Orlando, FL 32826, USA.

and prevent the dendrite-induced short circuit of ZABs. During the discharge process, the Zn anode will lose two electrons and be oxidized to Zn 2+ , which then react with OHin the alkaline electrolyte to form Zn(OH)4 2-(Eq. 1 and Figure 2b). Zn(OH)4 2will be decomposed to insoluble and semiconducting Zn oxide (ZnO) when the concentration reaches supersaturation in the electrolyte (Eq. 2 and Figure 2c). In the meantime, two liberated electrons from the anode transfer to the air cathode through the external circuit and reduce O2 to hydroxide ions (OH -) at the cathode (ORR, Eq. 3). Overall, a theoretical potential of 1.65 V (Figure 1a) can be delivered. When recharging ZABs (reverse reactions of Eq. 1-3), the deposition of Zn from Zn 2+ dissolved in the electrolyte occurs on the metal anode and OER occurs at the air cathode. Zn + 2 2 → ( ) 2 + 2 ( ) (5) ( ) 2 → + 2 (6)

Strategies for alleviating the surface side-reactions
In terms of cell design engineering for ZABs, Zn-air flow cells can be considered to reduce the surface side reactions with the help of a flowing system compared with the stationary ZABs. [6d, 20] In the ZABs flow cells, recycling the fresh anode alkaline solution (dispersed metallic Zn slurry) or the fresh alkaline aqueous electrolyte to the reactor is an efficient way to alleviate the surface sidereactions. The flowing system will not only help improve the discharge/charge efficiency of the Zn anode but also avoid the formation of the by-product from side reactions, thus achieving favorable O2 access due to less cathode clogging. Also, the structure of ZABs flow cells with the flowing electrolyte can inhibit the Zn dendrite growth effectively compared with the stationary ZABs without the flowing system. Consequently, the dendrite-free morphology of the Zn anode was observed in the flowing electrolyte instead of the stationary electrolyte. [21] Thus, the ZABs flow cells exhibit unparalleled battery performance, including superior power output and excellent cycling stability. Some representative works about ZABs flow cells showing significantly improved battery performance, stability, and efficiency than the stationary ZABs have been reported recently. [6d, 20e-h, 22] While the ZABs flow cells need the supernumerary tubes, pumps, and excessive electrolyte, increasing the system complexity and cost. Also, the external power pump is necessary for the ZABs flow cells to drive the liquid flow, which may consume extra energy. [6d, 20a] Besides, from the electrochemical and material science aspects, other strategies including increasing surface area, fabricating 3D electrode structure, adding additives to the electrodes/ electrolytes, and electrode coating are also used to improve the ZABs cyclability. [11a, 23] The

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This article is protected by copyright. All rights reserved increased surface area of the Zn anodes will facilitate the surface reaction kinetics, thus mitigating the Zn dendrite formation during the charging process. It should be noted that with the increased surface area of the Zn anode, the reaction rate of HER will also increase due to the exposure of active sites. To improve the mechanical stability and alleviate the shape reconstruction of the Zn anode, the polymeric binders, such as polytetrafluoroethylene (PTFE), [24] carboxymethyl cellulose (CMC), [11a, 24b] agar, [25] [26] , and poly(vinylidene fluoride) (PVDF) [26][27] are added to the anodes.
The binders can make much better dispersion of Zn and ZnO powders, which inhibits the dendrite growth of Zn and increases the effective surface area. Besides, due to the good conductivity and chemical stability in the alkaline environment, some carbon-based additives (i.e., carbon black) are used on the Zn anode to improve the utilization efficiency of Zn and avoid surface passivation. The use of heavy metals (i.e., Bi, In, Pb, Cd, Ti, Sn, etc) and their compounds (oxide, hydroxides, and nitrides) additives [28] is also an effective route to improve the conductivity and current distribution of the Zn anode. The heavy metals can maintain their metallic phases while the active Zn have been discharged and converted to insulating ZnO. These heavy metals usually possess much higher overpotentials than Zn/ZnO for HER, thus inhibiting the various surface side-reactions simultaneously. The HER on the Zn electrode can also be alleviated by reducing the H2O activity using a highly concentrated Zn-ion electrolyte (HCZI), [29] in which a unique solvation-sheath structure of Zn 2+ will be formed in the HCZI electrolyte, making the (Zn-TFSI) + much easier to be formed due to the huge amounts of anions forces and thus significantly suppressing the formation of (Zn-(H2O)6) 2+ . Therefore, the HCSI not only retains water in the open atmosphere, but also enables dendrite-free Zn plating/stripping at nearly 100% CE. Moreover, using a polymer electrolyte can further inhibit the HER on Zn electrode. [27,30] The dissolution and deposition of Zn and ZnO can be tuned by optimizing the structure of the Zn anode, thus minimizing the shape reconstruction of the Zn anode during long-term operation.
For example, Debra R. Rolison reported the use of a 3D Zn sponge anode in alkaline Zn batteries

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This article is protected by copyright. All rights reserved (Figure 3c). [31] The chemical reactions of Zn in the nickel-3D zinc batteries are similar to those in the ZABs. The developed 3D Zn sponge anode showed minimal shape reconstruction and no dendrite formation was observed, while severe dendrite was found to form on traditional Zn powder anode. A much higher utilization efficiency of Zn was obtained due to the monolithic structure and improved surface area of the 3D Zn sponge structure. Due to the novel 3D sponge structure, the well-distributed current and electrolyte, as well as superior performance, were obtained. Almost 90% of the theoretical capacity was achieved in the charging process, and more than 95% of the capacity can be recovered. However, as the increased surface area and reduced electrical conductivity of the 3D Zn sponge anode, the HER should be carefully addressed as we discussed above.
Other strategies, such as electrolyte additives (such as K2CO3, KF, K3PO4, and K3BO3), electrode coating with other materials have also been investigated. [32] The coating method should allow sufficient migration of OHions to facilitate the charge and discharge processes, but simultaneously reduce the migration rate of Zn(OH)4 2outward during the discharge process. The shape change of the Zn electrode can be mitigated and the concentration gradients can be reduced due to the inhibited Zn(OH)4 2migration during the charging process, which lowers the driving force for dendritic growth and significantly improves the cycle life of ZABs. [6a] For example, the Zn metal surface coated with Li2O-2B2O3 will not only suppress the HER but also increase the discharge capacity due to that the coating layer prevents the direct contact of the Zn surface with the strong alkaline electrolyte, thus avoiding the side reactions of the Zn electrode. [33] The corrosion resistance can be increased by coating with neodymium conversion films on the Zn electrode and thus stabilizing the cycle behavior of the Zn electrode. [32c] Recently, a porous nano-CaCO3 coating layer as a buffer layer which can lead the uniform and position-selected Zn stripping/plating on Zn foil interfaces was reported (Figure 3d), [34] The high porosity of CaCO3 can be readily permeated by the aqueous electrolyte, guiding a relatively uniform electrolyte flux and Zn plating rate over the

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This article is protected by copyright. All rights reserved Zn foil surface. The small-sized Zn nuclei would be confined in the nanopores of the CaCO3coatings, lowering polarizations of these electrodes. In addition, a large potential variation across the nano-CaCO3-coating was existed due to its electrically insulating nature. Hence, only the potential near the Zn foil surface was negative enough for Zn 2+ reduction, leading to a positionselected, and dendrite-free stripping/plating process, which delivers a 42.7% higher discharge capacity than bare Zn electrode even after 1000 cycles. It should be noted that the coating strategy, including inorganic and organic coating layers, is still limited in improving the performance and life of the Zn anode. The inorganic coating layer is brittle, which is easy to fracture under a long-time cycle and fast Zn plating/striping. The organic coating layer is flexible, however, the hydrophobic polymer layer will cause a drastic increase of polarization potential for Zn plating/stripping owing to the elevated nucleation barrier and restricted 2D diffusion of Zn. [35] Besides the coating method, another effective method to inhibit the dendrite growth and surface passivation is controlling the interfacial electrochemistry on the Zn/electrolyte interface. [36] The combination of the inorganic and organic composite coating layer (Nafion-Zn-X) method was also developed to take the advantage of coating and interface electrochemistry. [35] The composite protection layer formed in the interface of Nafion-Zn-X can not only shield anion and free H2O to suppress the side reactions, but also restrain the Zn dendrites by uniform Zn plating/stripping. A composite protective layer consisting of nanosized metal-organic frameworks (MOFs) and PVDF was also developed to improve the poor wetting effect of aqueous electrolytes on the Zn anode to reconstruct the Zn/electrolyte interface. [37] In the MOF-PVDF layer, the hydrophilic MOF nanoparticles serve as interconnecting electrolyte reservoirs, enabling nanolevel wetting effect as well as regulating an electrolyte flux on Zn anode. This zincophilic interface exhibits significantly reduced charge-transfer resistance. Thus, stable and dendrite-free Zn plating/stripping cycling performance is achieved for over 500 cycles.

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This article is protected by copyright. All rights reserved Recently, an electrodeposition approach was developed to fabricate a 3D Zn3Mn alloy anode to address the Zn dendritic growth and interfacial instability issues, which was proved to significantly improve the electrochemical performance of ZABs. [11b] As the more negative reduction potential of Mn/Mn 2+ than Zn/Zn 2+ , the electrostatic shield effect can help inhibit the Zn dendrite formation during the Zn deposition on the Zn-Mn alloy surface. Compared with the traditional Zn plate with a dendrite growth, the Zn3Mn anode surface with a homogeneous Zn coverage was achieved because that the proper bonding between Zn and Mn in the Zn3Mn alloy provides fast Zn diffusion channels and therefore suppresses dendrite growth (Figure 4a). The indepth study indicates that both the reaction kinetics and thermodynamics on the 3D Zn3Mn electrode can be well controlled. The minimal dendrite formation, Zn nucleation-growth at the initial stage of the plating can be guided and regulated by the electronic structure of the Zn3Mn alloy. As a proof of concept, the as-developed Zn3Mn alloy showed much better cycling performance, discharge capacity, and power density than the ZABs using commercial Zn anodes (Figure 4c-e). Also, it can work as flexible ZABs ( Fig. 4f-h), showing outstanding charge/discharge and cycling performance for ZABs.
Generally, the supersaturated Zn(OH)4 2will be decomposed to compact and insoluble semiconducting ZnO, which decreases the performance of ZABs (Figure 2c). Recently, a Zn peroxide (ZnO2) was proposed through a 2e -/O2 process in non-alkaline aqueous electrolytes. In contrast to the traditional used alkaline solution, the hydrophobic trifluoromethanesulfonate (OTf -) anion with a large molecule size was used as electrolyte solute, and the so-called Zn 2+ -rich and water-poor inner Helmholtz layer were established for the air cathode, thus promoting the aprotic 2e -ORR ( Figure 5). Compared with the widely used KOH electrolyte, the adopted OTfanionsbased electrolyte significantly increases the Zn utilization ratio (ZUR) by more than ten times (83.1% vs. 8.1%, Figure 5a). Besides, compared with the traditional KOH electrolyte in ZABs which was susceptible to the pH change and CO2 contamination from the atmosphere, the OTf -

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This article is protected by copyright. All rights reserved electrolyte-based ZABs show more sustainable and stable performance in air and no obvious change of pH during the charge/discharge cycling (Figure 5b-e). The ZnO2/Zn has high reversibility during the charge/discharge operation (Figure 5f). Different from the traditional aqueous KOH electrolyte with a 4epathway, a 2echemistry was obtained in this OTfelectrolyte ( Figure 5g). The Zn 2+ -rich, H2O-poor structure, and the weak attraction between Zn 2+ cations and OTf − anions are the two major reasons to promote 2e -ORR chemistry (Figure 5h). In future research, the ionic conductivity and viscosity of OTfshould be considered to improve the rateperformance and power density of ZABs.

Chemistry and materials of the air cathode
When discharging ZABs, the O2 from the atmosphere diffuses into the air cathode and be reduced to oxygen-containing species (OH -, Eq. 3) through ORR. For the primary ZABs, the consumed Zn anode must be mechanically replaced with a new Zn anode. With the development of ZABs, the rechargeable ZABs attracted much attention to reducing the cost of ZABs. The discharging process in the rechargeable ZABs will provide electricity, while transient energy such as wind energy and solar energy can be used to recharge the ZABs. Especially, the rechargeable characteristic is vital for electric vehicles. During the charging, the electrochemical process is the reverse reaction of Eq. 3 at the air cathode, in which the OER will proceed.
The performance of electrically rechargeable ZABs largely relies on the air cathode with bifunctional activity and durability, which will face harsh conditions during the repeated discharge/charge processes in aqueous alkaline electrolytes. Developing bifunctional ORR/OER catalysts are a great challenge due to the large overpotential between the ORR and OER processes.
The materials for air cathodes should be both chemically and electrochemically stable in a strongly alkaline solution. The electrocatalysts for air cathodes must keep high activity for ORR/OER under both highly reductive and oxidative environments under high current densities. A wide voltage window from 0.5 V to 2.0 V (pH = 14) is usually used in ZABs. Some reports are advocating that a

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This article is protected by copyright. All rights reserved three-electrode system (where the Zn anode is located between the ORR electrode and OER electrode) can be used to replace the two-electrode system, [8b], [38] in which the ORR and OER occur during the discharge and charge process, respectively. For the three-electrode system, the exposure risk of ORR electrocatalysts (or OER electrocatalysts) to the oxidative (or reductive) environments is avoided, and therefore mono-functional ORR and OER catalysts can be used, which will dramatically improve the batteries lifetime as well as the cycling stability. However, because of the relatively complicated cell design of the three-electrode ZABs, two-electrode ZBAs using bifunctional catalysts on the air cathodes are still dominating the research in the field. The proposed three-electrode system can be used for electrocatalysts assessment at the start-up stage, optimizing and enhancing the battery cycling stability.

Air cathode structure
The ORR/OER takes place at a gas-liquid-solid (gaseous O2, liquid KOH electrolyte, and solid catalysts) three-phase interface. Therefore, a boundary with a high surface area between the three phases is essential for highly active and efficient air cathodes. During ORR, the O2 diffusion from the atmosphere through the gas phase is much easier and quicker than that through the liquid electrolyte since the much lower solubility and diffusivity of O2 in most solvents (Figure 6a).
Traditional cathode structures for ZABs cannot provide enough three-phase interface zones to meet the requirements for ORR/OER occurring under the condition of high current density and continuous operation. The porous gas diffusion electrodes were used in the early stage of ZABs development. The surface/interface of the gas diffusion electrode consists of a hydrophobic gas diffusion layer (GDL) and a moderately hydrophilic catalyst layer (CL). The GDL guarantees the smooth diffusion of O2 from the air into the reaction interface, while the CL supported on the GDL catalyzes the ORR/OER. The GDL not only provides gas channels for O2 diffusion in and out during the discharge (ORR) and charge (OER) but also serves as conductive support for the catalysts. Furthermore, the GDL also acts as a wet-proofing layer to prevent the leakage of the

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This article is protected by copyright. All rights reserved aqueous electrolyte. An ideal air cathode should contain a hydrophilic CL and hydrophobic GDL.
Noted that suitable humidity is crucial for the ZABs performance. Low humidity will result in the gradual evaporation and drying-up of the electrolyte, while high humidity will cause the flooding of the air cathode. Therefore, the well-balanced hydrophilic and hydrophobic properties of the air cathode are very important for ZABs. Hence, the rational design of air cathodes with optimal hydrophilicity, and friendly interface structure is crucial to ZABs. Nowadays, the most widely used commercial GDL is designed and developed by Toray Industries, Inc (Japan). In the Toray carbon paper, the GDL with high porosity, and electrical conductivity was acquired, in which the carbon fibers are tied together by graphitized carbon layers. After treated by wet-proofing agents PTFE, the Toray carbon paper can be used for various research purposes (Figure 6b). Even though the Toray carbon paper (carbon cloth) has shown big success as GDL, especially in the proton exchange membrane (PEM) fuel cell research field, however, it should be kept in mind that the ZABs need to be operated under harsh conditions. Apart from the ORR, the OER needs a much higher working voltage (generally above 2.0 V), which is higher than the carbon oxidative corrosion voltages. [39] Thus, it is urgent to develop cheaper and alternative GDL, which should possess good anticorrosion property, excellent electrical conductivity, and high surface area.
Another important issue for ZABs is that the diffusion of CO2 and H2O from the atmosphere into the air cathode together with O2. The CO2 will react with the alkaline electrolyte and thus form carbonate precipitation, which can block the diffusion/transport channels. Additionally, the H2O can also dilute the electrolyte and change the pH of the electrolyte, affecting the ZABs performance.
Therefore, managing the CO2 and H2O diffusion through a selective membrane only allowing O2 diffusion but excluding/blocking CO2 and H2O is feasible (Figure 6c-d). [40]

Bifunctional ORR/OER catalysts
The most efficient ORR and OER electrocatalysts have been developed using scarce and expensive platinum metal groups (PGMs) materials (i.e., Pt, Pd, IrO2, RuO2, etc). The Pt and IrO2

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This article is protected by copyright. All rights reserved At the early stage of ZABs development, the PGMs were firstly used and studied to lower the overpotential and increase ZABs efficiency. In general, the four elementary reactions for ORR can be expressed as below (Eq. 7-10, reversed processes for OER).
Theoretically, the volcano plots (Figure 7) have been widely used to present the scaling relations of the binding energies of the reactants on different catalytic surfaces. Both the theoretical and experimental results indicate that Pt, Pd, and Ag are among the best metals for ORR, sitting almost on the apex of the volcano (Figure 7a). However, even for the most reactive catalyst with the

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This article is protected by copyright. All rights reserved optimal adsorption strength at the peak of the volcano plot, an overpotential of 0.3~0.4 V was still found (Figure 7a). Based on the scaling relation, two effective strategies can be used to further boost the ORR activity (Figure 7b), including tunning the d-band electronic structure (strain, surface orientation, and composition) and regulating the intermediates' adsorption sites (coordination environments). [41] Due to the high charging voltage (usually around 2 V), most of the OER catalysts are metal oxides (Figure 7c). The OER also goes through a four-electron process and has a general adsorption-energy scaling relationship on the surface of oxides, including spinel, perovskite, and bixbyite oxides, etc. Like ORR, the best OER catalyst still shows overpotentials from the scaling relation ( Figure 7c). The OER performance can be further boosted through various strategies, such as morphology fine-tuning, crystal facet control, composition regulation, defect, strain, and doping engineering. [42] For example, the hierarchical mesoporous Co3O4 nanowire array possesses much higher bifunctional OER/ORR activity than other morphologies. [43] The α-MnO2 with nanospheres and nanowires morphologies has a much better OER performance than that with microparticles morphology due to the smaller particle size and higher specific areas. [44] The Co3O4 with exposed (111) planes has a much higher OER performance than that with exposed (001) planes due to the much lower O2 desorption activation barrier. [45] Regulating the elemental composition of A-site or B-site in perovskite (ABO3 structure) also can boost the OER performance due to the partial reduction of Ni 3+ to Ni 2+ as a result of oxygen vacancies (OVs). [46] The bandgap will be narrowed on account of the donors of the OVs, thus increasing the electron density and electrical conductivity of materials. In addition, the surface of OVs can result in the enhancement of the electron transfer from O-vacancies to metal d-band, thus tuning the adsorption of surface species for OER/OER. [47] As proved by the previous study, [48] the OER activity of perovskite oxide has a volcano relationship with the numbers of eg electron. Thus, the degree of eg orbital splitting and polarization can be adjusted by the strain engineering [49] . The heteroatom doping, (N, P, S, B, et al), into carbon results

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This article is protected by copyright. All rights reserved in multiple possible configurations, making the neighboring carbon atoms electron-deficient, thus contributing to the oxygen adsorption. [50] Although great efforts have been made for the development of ORR and OER electrocatalysts, the bifunctional catalysts are inherently not the best option due to the separated vertices of the ORR/OER volcano plots (Figure 7d). However, due to the essential role of the bifunctional catalyst, both the OER and ORR linear scaling relation derived from volcano plots should be considered in the future (Figure 7d). We will summarize the bifunctional catalysts as below:

PGMs and their alloys
PGMs such as Pt have been widely studied as air cathodes in ZABs. The Pt catalysts anchored in high surface area carbon (Pt/C) were physically mixed with OER catalyst IrO2 (RuO2) and used as standard air cathodes for ZABs. The scarcity and high cost of these PGMs urge the researchers to develop effective approaches to maximum its activity and stability with reduced loading, [51] such as reducing the dimensions of Pt down to nano or even atomic scales and tunning the morphology and crystal facets. [52] Another effective and viable strategy is alloying Pt with other less-expensive noble metals (such as Pd), [53] heavy metals (such as Pb), [54] and transition metals (such as Fe, Ni, Co, Mo, and Cu). [55] Recent studies indicate that the Pt3Ni (111) alloy possesses an uncommon electronic structure, where the highly structured compositional oscillation was found in the near-surface region. It is found that the outermost and third layers of Pt3Ni (111) are Pt-rich, while the second layer is Ni-rich.
[55e] The Pt3Ni (111) surface exhibits more than ten times higher ORR activity than the Pt (111) surface. Also, a relationship between the surface composition, electronic structure, and the ORR specific activity of Pt3M (M = Fe, Co, Ni, V, and Ti) was established by Markovic. [56] They found that the Pt3Co and Pt3Ni possess the best ORR performance, which is in line with the results shown in Figure 7b.
Some significant progress in Pt (Pd)-based materials have been made for ORR in hydrogenoxygen fuel cells, while it is rarely reported for OER due to the formation of surface oxide layer

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This article is protected by copyright. All rights reserved during charging process under high voltage. Recently, a novel method to immobilize Pt atoms in platinum cobalt (PtCo) alloy nanosheets was developed by combining electrochemical deposition with fluorine-plasma etching treatment (Figure 8a). [55g] The atomic Pt were stably anchoring on the surface of PtCoF due to the lattice distortion induced by interstitial F doping (Figure 8b-d). The The optimization of surface strain and coordination environment in the alloys also play a vital role in boosting the bifunctional ORR/OER performance. [57] Recently, Guo's group reported superthin PdMo bimetallene nanosheets (Figure 9a). [55f] As Mo has a larger atomic radius than Pd, this PdMo bimetallene has a higher lattice parameter than Pd. A tensile strain of 0.95~1.4 % was obtained due to the presence of Mo and the curved geometry (Figure 9a). DFT calculation indicates that the optimal oxygen adsorption energy (ΔEO) can be obtained when the tensile strain is ca. 1 % (Figure 9b-c). The d-band of the surface Pd atoms will be filled and shift towards negative energy with charge transfer from Mo to Pd surface (Figure 9d) (Figure 9e-f).
Other Pt-, Pd-, and Ag-based materials were also in-depth studied and reported as air cathodes over the last 4 years. [58] However, bear in mind that the high price and scarcity of these PGMs severely hinder their large-scale applications in ZABs. The development of PGMs-free catalysts is necessary and will promote the commercial application of rechargeable ZABs.

Carbon-supported inexpensive metals and alloys
The ORR/OER processes are more favorable to occur in the alkaline environments, showing faster kinetics, lower overpotentials, and much higher exchange current density than those in the acidic environments. [59] According to the Nernst equation, the electrode potential will shift negatively about 0.83 V if the electrolyte changes from acid (pH = 0) to alkaline (pH = 14). The negatively shifted potential has a strong effect on the electric field at the interface between electrode and electrolyte, thus forming weak binding strengths between adsorbates and charged species. [60] Thereby, the electrocatalysts show greatly weakened ions adsorption in alkaline media than in the acid, and exhibit more facile electrocatalytic processes in alkaline solutions. These make it possible that using the non-PGMs as bifunctional electrocatalysts for ORR/OER. Due to the low price, high surface area, and good electrical conductivity of carbon materials, carbon-supported electrocatalysts are the most widely used materials for ORR/OER. We will introduce the most developed carbonsupported inexpensive metals and alloys as efficient ORR/OER bifunctional electrocatalysts as below: The porous structure and sufficient active sites/areas of carbon-supported catalysts are essential for electrocatalytic ORR/OER reactions. A template-free strategy to synthesis porous

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This article is protected by copyright. All rights reserved carbon nitride (PCN) based CoSx@PCN/rGO bifunctional catalysts was recently developed. [61] This template-free and top-down strategy can be easily controlled (Figure 10a). Due to the porous structure and highly exposed active sites, the CoSx@PCN/rGO shows excellent ORR/OER bifunctional catalytic activities with the potential gap of 0.79 V between the OER (at 10 mA cm -2 ) and the ORR (half-wave potential, Figure 10b), which is much lower than that of commercial Pt/C (1.10 V) and RuO2 (1.08 V). The CoSx@PCN/rGO can be used as an efficient bifunctional catalyst for ZABs (Figure 10c-d), displaying excellent cycling stability without performance decay for over 400 discharge/charge cycles. Thus, the CoSx@PCN/rGO shows the potential to replace PGMs as an efficient air cathode for ZABs.
The morphological emulation, such as human hair array, [13c] pomegranate-inspired structure, [62] hydrangea-like superstructure, [63] and overhang-eave carbon cages, [64] also offers a way for developing efficient air cathodes. Recently, an apically dominant mechanism was proposed to improve the ZABs performance. [65] In the developed catalysts, the CoNi alloy nanoparticles were encapsulated in the apical domain of nitrogen-doped carbon (NCNT) nanotube on the Ni foam (CoNi@NCNT/NF). Compared with other alloy nanoparticles randomly loading on supports, the CoNi alloy was controlled to be mainly distributed on the top of NCNT (Figure 10e-f). In the CoNi@NCNT/NF, due to the hydrophobic nature of carbon surface, most of the ORR/OER intermediates can only approach the vertex rather than the base of NCNT arrays, which will provide ideal reaction spaces for ORR and OER. The ZABs coin cells were assembled using the CoNi@NCNT/NF as air cathodes, showing a maximum power density of 127 mW cm -2 and energy density of 845 Wh kgZn -1 . Furthermore, the ZABs using CoNi@NCNT/NF can operate stably for over 90 h with contineous charge/discharge cycling (Figure 10g-i).

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This article is protected by copyright. All rights reserved positive charge will be created around carbon atoms, which can improve the oxygen chemisorption and electron transfer, thus resulting in the enhanced catalytic activities for ORR/OER. [74] N can exist in the form of graphitic, pyridinic, pyrrolic, and oxide forms when incorporated into the graphitic network. [75] Recently, Dai's group reported a 3D structured N-doped graphene nanoribbon (N-GRW), [76] which contains both p-type and n-type N dopant configurations. The N-GRW was used as catalysts for ORR and OER. From the XANES spectra of N K-edge, they found that the quaternary N peak after ORR was formed, while the intensity of pyridinic N peak increased after OER (Figure 11a). The n-type doping quaternary N were identified as the active sites for ORR, while the p-type doping pyridinic N in the N-GRW acted as active sites for OER (Figure 11b). The carbon materials doped with other heteroatoms also show good ORR/OER performance, such as P, S, and Se (a bigger atomic radius and more electronegative than C), B, F (a smaller atomic radius and less electronegative than C). In addition to single heteroatom doped carbon, the binary heteroatoms doped (such as N/P, N/S, N/F, N/P) and ternary heteroatoms doped (such as N/P/B and N/F/B) carbon materials also have been investigated to improve the ORR/OER performance. In future research, more advanced techniques should be developed to precisely identify the active sites of these heteroatoms doped carbon materials.
Dai's group also reported N and P co-doped mesoporous carbon foam (NPMC). [77] The NPMC with a super high surface area (1,663 m 2 g -1 ) was prepared by pyrolysis of polyaniline aerogel containing phytic acid (Figure 11c). The pyrolysis temperature is very important because that the relatively low pyrolysis temperature can result in high charge-transfer resistance. While the overheating at high pyrolysis temperature will lead to the removal of doped heteroatoms (N and P), resulting in the reduced active sites. They found that the optimal temperature is 1000 o C, at which the moderate graphitization degree, electrical conductivity, surface area, and N/P content can be well kept. The metal-free NPMC can be used as air cathodes for both primary ZABs (Figure 11d-f) and rechargeable ZABs (Figure 11g-i). Specifically, the OCP of primary ZABs can be as high as

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This article is protected by copyright. All rights reserved 1.48 V with a peak power density of 55 mW cm -2 . The specific capacity and energy density of 735 mAh gZn -1 and 835 Wh kgZn -1 (Fig. 11d-f), respectively, were achieved and stably operated at 2 mA cm -2 for 240 h with just replacing new Zn anode (mechanical recharging). It shows stable performance for 600 cycles (100 hours) in rechargeable ZABs at 2 mA cm -2 in three-electrode ZABs (Figure 11g-i). According to the DFT calculations, it is found that the synergistic electronic interactions are generated by N, P dopants, and adjacent carbon atoms help improve the electrocatalytic bifunctional OER and ORR activities.
Based on the above discussions, the modified carbon-based materials are widely used as support/active materials for air cathodes in ZABs due to their high conductivity, porosity, and high specific surface area. However, these carbon materials suffer from serious and rapid structure decays and are easily oxidized to carbon monoxide/carbon dioxide under anodic oxidizing conditions under the high applied voltage during the charging process. This is bound to result in the poor stability and inferior circulation of air electrodes during the charge/discharge process in rechargeable ZABs, and in turn, reduce the catalytic activities due to the collapse of the carbon structure and the dissolution/aggregation /de-metalation of the metals on the carbon supports.
Although the increased graphitization degree of carbon materials can inhibit the corrosion to a certain extent, the lifetimes of carbon materials can still be significantly reduced due to the electrochemical corrosion with the strong alkaline solution. Thus, exploring materials with anticorrosion, high electrical conductivity, and high surface area is necessary. The metal foam/mesh (such as Ni and Cu) seems to be effective alternative supports (Figure 6c). In all, much effort should be paid attention to these carbon corrosion concerns.

Carbon-free catalysts
As stated above, high operation potential is needed during the charging process in ZABs.
While the standard thermodynamic potential for carbon oxidation to CO2 under alkaline solution is 0.621 V (vs. SHE, Eq. 11), [78] which is much lower than the charging voltage of ZABs (usually

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This article is protected by copyright. All rights reserved exceed 2.0 V). Thus the carbon-based materials are easily oxidized, and most of the OER catalytic materials are metal or metal oxides (Figure 7c).
C + 4 − → 2 + 2 2 + 4 − ( 0 = 0.621 . ) The porous metal (alloy) films, spinel, rutile, perovskite have been widely studied due to the low cost, high abundance, and environmental friendliness. To solve the shortcomings of carbonbased materials that being easily oxidized and corroded in electrochemical ORR/OER environments, a novel strategy combining electrodeposition with electroetching was reported to prepare an additive-free nickel sulfide (NiSx) freestanding holey film (NiSx FHF, Figure. 12a). [13h] The NiSx with porous structure has an optimal electrochemically active surface area and active sites, which is a good candidate as stable bifunctional ORR/OER catalysts and can be directly used as air cathode.
The stability and potential application of the flexible rechargeable ZABs using NiSx FHF were assessed by the galvanostatic discharge curves with different deformation angles at a current density of 2 mA cm -2 . Even with different deformation angles, the flexible ZABs show no obvious performance decay (Figure 12b), thus validating the potential application of NiSx FHF as air cathodes for flexible ZABs. Besides, galvanostatic charge-discharge curves further confirm the much better stable performance of NiSx FHF than benchmark Pt/C-RuO2 electrodes (Figure 12c).
This work provides an effective strategy to prepare carbon-free, PGMs-free, and non-additive freestanding porous electrodes for ZABs.
As discussed above, apart from boosting the performance of carbon-based materials, heteroatoms doping strategy can also be applied to the metal-based materials to enhance the conductivity, increase the active surface area, and facilitate reaction kinetics. For example, the N, Pcodoped CoS2 nanoclusters embedded inside TiO2 nanoporous films (N, P/CoS2@TiO2 NPFs) were developed, [79] as shown in Figure 12d. After the anodic oxidization treatment, TiO2 NPFs with a highly ordered honeycomb morphology were obtained, which were subsequently coated by Co and processed sulfurization treatment to construct a high active bifunctional ORR/OER catalysts.

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This article is protected by copyright. All rights reserved Compared with the benchmark Pt/C and other control samples, the N, P/CoS2@TiO2 NPF shows the lowest voltage gap between E1/2 for ORR and Eonset (10 mA cm -2 ) for OER (0.78 V, Figure 12e).
The as-prepared N, P/CoS2@TiO2 NPFs can be directly used as air cathodes for flexible ZABs. The galvanostatic discharge curves of flexible ZABs using N, P/CoS2@TiO2 NPFs at different deformation angles show no obvious performance decay at 10 mA cm -2 , indicating the great potential application in flexible devices (Figure 12f). Moreover, compared with the gradual performance decay (the voltage gap was increased over time) of ZABs using Pt/C-RuO2 as air cathodes (Figure 12g-h), the N, P/CoS2@TiO2 NPFs-based ZABs also showed strong stability even with a continuous 200 cycles (133 hours) test (Figure 12i-j). Furthermore, the specific capacity of 610 and 580 mAh g Zn -1 was obtained at a discharge current density of 10 and 50 mA cm -2 . From the in-depth analysis, it is found that the heteroatoms play different roles in the N, P/CoS2@TiO2 NPFs, that is, N-doping was mainly used to increase the conductivity and electrochemical activity of the NPFs, and the P-doping can provide the electrode a passivated surface and thus improve stability. Thus, this N, P/CoS2@TiO2 NPFs show outstanding ORR/OER performance for ZABs.
The free-standing metal porous films can not only be used as catalysts, but also as support to replace carbon materials due to their good conductivity and corrosion resistance. Recently, a porous FeCo glassy alloy film was developed as bifunctional catalytic support to replace conventional carbon supports. The conducting framework of the porous FeCo glassy alloy was used to stabilize ORR/OER cocatalysts, in which the FeCo were the main active sites for OER, and the ultrasmall Pd nanoparticles anchored on the FeCo glassy alloy as ORR active sites. A half-wave potential of 0.85 V (vs. RHE) for ORR was acquired for the Pd/FeCo, which is almost the same as Pt/C. Also, it only needs 1.55 V (vs. RHE) to reach the current density of 10 mA cm -2 for OER (Figure 13a). The Pd/FeCo can be directly used as the air cathode in rechargeable ZABs, exhibiting a maximum power density of 117 mW cm -2 (Figure 13b) and no obvious performance attenuation after

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This article is protected by copyright. All rights reserved continuous operation for 400 cycles (200 hours, Figure 13c). This work offers a new strategy to design novel carbon-free self-supported materials for ZABs. For example, Jaramillo reported a simple and effective electrodeposition approach together with a thermal treatment in the air to prepare nanostructured MnOx thin-film materials on glassy carbon. [81] The MnOx thin film shows comparable ORR and OER performance to the commercial Pt/C, Ir/C, and Ru/C. For the MnO2, it has three crystal structures, and the catalytic ORR activities increase in the order of γ-MnO2 < β-MnO2 < α-MnO2. [82] One trend of the Mn-based materials for ORR is that the catalytic activities correlate with the nanostructures, exposed facets, and Mn valence. While much more efforts should be taken to explore the in-depth structure-valence-performance relationship of Mn-based materials.
Co3O4 has been widely used as OER catalysts, [83] however, its ORR performance is poor and rarely studied. Coupling Co3O4 with other materials, such as Ni, Mn, and Pd offers an effective method to improve ORR performance through electronic effect. [84] For example, Chen's group reported chemical deposition of Pd nanoparticles on the 3D ordered mesoporous Co3O4 (Pd@3DOM-CO3O4, Figure 13d). [85] The d-band center of Pd was decreased while the Fermi level was increased due to the role of 3DOM Co3O4, which improved the overall kinetics and conductivity in ZABs. The Pd@3DOM-CO3O4 was employed as an air cathode in rechargeable ZAB, showing long-term stability of 300 cycles (50 hours, Figure 13e).
Other bifunctional oxides were also studied, such as spinel (AxB3-xO4) structure and perovskite (ABO3) structure. [86] All these materials can be used as air cathodes due to their high catalytic

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This article is protected by copyright. All rights reserved activity for ORR/OER and excellent anti-corrosion property in alkaline solution. Recent studies indicate that the ORR activities of perovskite materials are related to the s*-orbital (eg) occupation and transition metal-oxygen covalency, which can be used as activity descriptors (Figure 7c). [87] Kim et al reported a mesoporous nanofibers cation ordered perovskite, PrBa0.5Sr0.5Co2-xFexO5+δ (x = 0, 0.5, 1, 1.5, and 2), in which the B-site metal ratios and the surface of mesoporous nanofibers can be well controlled (Figure 13f). This cation-ordered perovskite showed high bifunctional activities for ORR/OER and good stability in ZABs (Figure 13g). Chen et al reported a bifunctional ORR/OER catalyst with a core-corona structure (CCBC) consisting of LaNiO3 and N-doped carbon nanotubes. [88] In these CCBC materials, the core was responsible for OER while the corona was designed to catalyze ORR. When tested in ZABs at 17.6 mA cm -2 a voltage gap of 1.3 V was achieved. Other types of perovskites, such as La1.7Sr0.3NiO4, LaMO3 (M=Ni, Co, Mn), and Ba0.6Sr0.4Co0.79Fe0.21O2.67 were also reported for ZABs. [89]

Summary and Perspectives
In this review, we systematically summarize the recent progress in electrode design from perspectives of electrochemistry and material science. The problems on the Zn anode, such as dendritic growth, shape reconstruction, passivation, and HER, were critically reviewed, and some effective solutions were proposed. For the air cathodes, the advantages and limitations of different types of bifunctional ORR/OER electrocatalysts, including PGMs, carbon-based materials, and carbon-free materials, have been summarized in Figure 14 and Table 1. The electrocatalytic reaction mechanism, influential factors, and electrode performance of ZABs were emphasized in this review. Although great efforts and some outstanding research achievements have been accomplished, the development of ZABs remains confronted with severe challenges.
(1) New working mechanisms for ZABs. Many fundamentals of electrochemistry and material science still need thorough understanding. For example, the 4etransfer process of Zn anode was recognized as the best choice with high efficiency at the beginning of the ZABs study. While the

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This article is protected by copyright. All rights reserved ZnO2/Zn with a 2etransfer process was proposed recently, [90] which show better reversibility during continuous charge and discharge process. In future studies, more strategies should be developed to improve the rate performance and power density of ZnO2/Zn batteries. Also, it is highly recommended to have a further understanding of the underlying mechanism of Zn redox chemistry. Besides, the dendrite growth, self-discharge, HER on the Zn electrode should also be addressed. Although some effective strategies such as using flow cells to mitigate the anode issues, the flowing system cannot be used in flexible devices. In addition, extra tubes, pumps, and excessive electrolytes are needed for flow cells. Extra electrical energy is also needed to drive the pump, which will not only increase the system complexity, volume, and cost but also decrease the energy efficiency. From material science aspects, increasing Zn anode surface area and fabricating 3D Zn structure have been used to improve the ZABs cyclability and Zn utilization efficiency.
However, the increased Zn surface area will lead to severe HER on the electrode, which further reduces the performance of ZABs. Thus, the development of novel Zn structure, architecture, and alloy strategies are highly desired, and new mechanisms for ZABs could be further investigated.
(2) New electrolytes. Although the alkaline aqueous electrolyte-based batteries show the best activity under ZABs operation conditions, the possible leakage and evaporation of alkaline electrolytes will corrode human skin and instruments when used for flexible electronics.
Additionally, the ZABs performance is largely dependent on the pH of the electrolytes. As the most commonly used electrolyte is a strong alkaline, it is recommended to real-time monitor the pH changes in the ZABs operation, especially during the charge and discharge process. Current experimental methods only report the pH of electrolytes before and after the operation, while important and useful information during the operation is not easily accessed. The pH value of electrolytes can determine the surface change, structure transformation, active site formation/evolution/depopulation of Zn anodes and air cathodes. Thus, real-time monitoring of the electrolyte pH is important for designing electrode materials. Besides, a solid-state electrolyte is

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This article is protected by copyright. All rights reserved preferred in flexible ZABs. However, the much lower ionic conductivity of solid-state electrolytes than aqueous electrolytes results in the inferior performance of flexible ZABs, and it is difficult to real-time monitor the pH changes for the solid-state electrolyte. In summary, the development of solid-state electrolytes and ionic liquids with low price and high ionic conductivity must be considered when extending the practical application of flexible ZABs.
(3) Water consumption in aqueous electrolyte. Most of the alkaline aqueous electrolytes are prepared by deionized water and freshwater on a laboratory scale. However, the freshwater resources shortage currently urges us to search for alternative water resources, such as seawater, low-grade and saline surface water, which is almost free and inexhaustible on earth. [11b, 91] However, for seawater-based ZABs, there are large numbers of anions (mainly Cl -) and cations (mainly Na + , K + , Ca 2+ , Mg 2+ ) in seawater. Besides, the biofouling and trace metal deposition also happen in seawater and saline water. Though the insoluble impurities can be removed by the filter, the Na + , K +, and Clcan't be removed and are still existed in seawater. The Clions will result in chlorine evolution reaction (ClER), [92] which is a competitive reaction of OER during the charging process due to the very close thermodynamic potentials for OER and ClER. [93] Thus, the seawater purification and development of catalysts with high selectivity for OER will be recommended. [94] The effects of both the anions and cations on the seawater-based ZABs performance should be carefully studied.
(4) CO2 and H2O managements. During the discharging process, the cathode materials need to react with O2 from the atmosphere for ORR. During the air diffusion into the air cathode, CO2 and H2O vapor will also diffuse into the electrolyte. Consequently, CO2 will react with alkaline electrolytes and thus generate carbonate to block the active sites of electrocatalysts and gas diffusion channels. And H2O vapor will change the pH value of the electrolyte. Both CO2 and H2O vapor should be filtered out before the ambient air diffusing into the electrolyte. Accordingly, adding a membrane to the air cathode to permit O2 diffusion selectively instead of the incorporation

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This article is protected by copyright. All rights reserved of CO2 and H2O is a feasible approach to control CO2 and H2O diffusion (Figure 6c). Some researchers also recommended adding a ''scrubber'' using cheap hydroxides (such as Ca(OH)2, soda lime, and amines). [95] Nevertheless, all the strategies will increase the system cost and complexity. Developing CO2-and H2O-resistant electrolytes and using acidic/neutral electrolytes should be considered. [96] (5) Surface wettability of GDL and CL. To avoid water flooding and keep the well-controlled O2 diffusion, a hydrophobic GDL should be used. While at the same time, to enlarge the threephase reaction interface and guarantee the intimate contact of gas-electrolyte-catalyst, a hydrophilic CL is also needed. The well-balanced hydrophobic and hydrophilic properties of GDL and CL should be seriously considered. The PFTE treated carbon paper (or carbon cloth) is a good choice for GDL, while the serious electrochemical oxidation corrosion of carbonaceous materials under high applied potential (> 2.0 V) will result in the gradually poisoned and deactivated carbon paper, leading to performance decay of ZABs. The porous metallic films and foams, foils, and sponges can be used as succedaneums for carbon-based materials, which have good conductivity and stability under corrosion potential. However, the surface /interface of these metal films, foams, foils, and sponges should be further employed by hydrophobic treatment to inhibit electrolyte flooding. Thus, the rational design of the air cathodes with optimal hydrophobic/hydrophilic properties and friendly interface structure/architecture is important.

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This article is protected by copyright. All rights reserved the reported rechargeable ZABs can only be stably operated from tens of hours to hundreds of hours, and only a few works can be operated for more than 1000 hours ( Table 1). The flexible ZABs using solid-state electrolyte shows much faster performance decay due to the gradual consumption of electrolyte even operated under a small current density. These performance of reported ZABs are far from the practical application (typically at least thousands of hours or even tens of thousands of hours). Thus, developing air electrodes with high-performance, cost-effectiveness, and anticorrosion is highly desired. The porous metals and alloys are good options for ZABs, which can be used as GDL and CL simultaneously. However, the balanced hydrophobic/hydrophilic properties, cost, usage amount, and surface area should be studied.
Overall, the excellent superiorities of safety, high energy density, and low cost allow the ZABs as a green and low-carbon energy storage and conversion technology. Much more scientific and industrial efforts on the developments of ZABs are highly encouraged. The ZABs will be ultimately used in many fields including wearable and portable devices.
This article is protected by copyright. All rights reserved  hours (2100 cycles) [107] Accepted Article This article is protected by copyright. All rights reserved hours (55 cycles) [114] Zn foil PAM-co-PAA (300 cycles) [118] 3D 2C rate, 250 cycles [120] Accepted Article This article is protected by copyright. All rights reserved C/5 rate, 50 cycles [121] In-doped ZnO cycles [37] Accepted Article This article is protected by copyright. All rights reserved