Aqueous Rechargeable Zn–Air Batteries for Sustainable Energy Storage

3.1.1 Gas-Diffusion Layer

Traditional air cathodes employ hydrophobic polytetrafluoroethylene (PTFE) binders to fabricate carbon-based GDLs [33]. While these GDLs promote O₂ diffusion during discharge, they hinder ion transport and retard OER during charging due to strong oxygen adsorption, limiting charge kinetics [34, 35]. Efficient OER requires hydrophilic channels in addition to hydrophobic pathways for ORR. Hydrophilic surface will allow H₂O adsorption and facilitate OER, while a hydrophobic/aerophilic microenvironment will facilitate (a) O₂ bubble detachment from GDL and (b) O₂ diffusion towards the cathode during ORR subsequently [36-38]. Introducing a hydrophilic polyvinyl acetate/carbon black layer between Pt/C + IrO₂ catalyst and PTFE-based GDL allowed O₂ release during charging via aerophobic interface (contact angle ~153°, Figure 2a), reducing charge voltage by 190 mV and improving energy efficiency by 6% [38]. Similarly, an expanded PTFE (ePTFE) air cathode facilitated gas transport through a “positive bubble diode” mechanism [41], with relatively hydrophobic/aerophilic front (100°) and more hydrophobic/aerophilic back (125°) (Figure 2b) [39]. Bubble diode mechanism is based on the Janus system (unidirectional transport of underwater gas bubbles, i.e., bubble-diode) in fluid dynamics, assisting O₂ bubble management and transport during RZAB charging (bubble penetration from less aerophilic side to more aerophilic side) [39, 42]. In contrast, conventional Ni foam and carbon current collectors show reverse bubble diode effect (hydrophobic front facing electrolyte), restricting O₂ bubble release [43].

The ePTFE electrode eliminated bubble accumulation due to optimal wettability, enabling rapid O₂ diffusion (Figure 2c) [39]. Mitigating electrolyte flooding also enhances cathode kinetics, where in catalyst distribution within GDLs rather than surface coating, preserves the triple-phase boundary and improve discharge kinetics and cycling stability (Figure 2d) [44]. This approach prevents the destruction of porous structure from traditional fabrication procedures (e.g., pressing, sintering) [45]. Solid/semi-solid electrolytes further address flooding but require optimized cathode wettability for practical performance [26, 46]. Other GDL modifications include (a) heat treatment to control hydrophobicity [47], prevent flooding and extend cycle life through modification of functional groups and carbon structure [48, 49]; (b) hierarchical structures, such as porous curly graphene films (PCGF), which improved GDL conductivity and microporosity, yielding 72% round-trip efficiency with Pt/C@PCGF cathode (Figure 2e–f) [40]; (c) use of high surface area substrates (e.g., Ni foam, carbon felt, carbon paper) [50, 51]; and (d) novel fabrication procedures [52, 53]. Overall, cathode kinetics is governed by GDL thickness and porosity, balanced hydrophobic–hydrophilic channels, surface area utilization, and optimized coating/drying techniques to maintain conductivity and structural integrity.

3.1.2 Oxygen Bifunctional Catalysts

Bifunctional catalysts are crucial for enhancing RZAB performance. While benchmark unifunctional catalysts for OER (RuO₂/IrO₂) or ORR (Pt/C) offer high activity, they are practically limited by cost, scarcity, complex synthesis procedures and compromised stability [54]. Oxygen bifunctional catalysts offer considerable promise for RZABs, yet achieving high activity remains challenging due to different ORR-OER reaction pathways, distinct binding energies of key intermediates (OH*, OOH*), and interdependent OH⁻/O₂ adsorption energies [55]. Current bifunctional catalysts exhibit round-trip efficiencies below 70% due to large ORR-OER overpotentials, obliging continued efforts to enhance bifunctional activity. These large overpotentials arise due to complex multistep reactions, sluggish electron/proton transfer, insufficient catalyst sites, and high energy barriers for intermediate binding/adsorption [56]. Even the benchmark Pt, IrO₂/RuO₂ catalysts exhibit intrinsic overpotentials over 0.2 and 0.4 V for ORR and OER, respectively [57, 58]. Thus, a bifunctional catalyst with ample proton binding sites, appropriate electronic structure and moderate binding strength for ORR/OER intermediates is essential.

Noble-metal based: Noble-metal based catalysts exhibit high ORR/OER activity by inducing lattice strain, modulating local electronic structures, exposing high-index facets, and regulating adsorption energies of oxygen intermediates [59]. However, they present poor stability due to agglomeration/deformation during prolonged cycling, while their high cost, scarcity, and complex synthesis inevitably increase manufacturing costs, hampering practical use (Figure 3a). Significant approaches to address these issues include partial substitution with transition metals (TMs) [31, 63, 64], incorporation into supports for improved stability [59], and single-atom catalysis [65, 66] for maximum atomic utilization. But achieving high stability and retaining intrinsic activity while minimizing noble-metal content remains challenging. Supported noble-metal catalysts (Figure 3b) on carbon materials (e.g., graphene, carbon nanotubes, carbon fibers, micro-/meso-porous carbons) [67-69] enhance conductivity, prevent aggregation, and tune electronic distribution. Though carbon corrosion under high oxidative potentials necessitates TM-supports (oxides, hydroxides, chalcogenides, phosphides) [70-72], providing not only stability but also OER active sites. Chameleon-like Ru single-atoms grafted on nickel-iron layer double hydroxide (Ru_SA-NiFe LDH) bifunctional catalyst demonstrated reversible ORR-OER (Figure 3c–d) through dynamic interconversion with Ru_SA-NiFeOOH, achieving 2400 h of cycling at 10 mA cm⁻² [60]. In future, developing multifunctional hybrid supports exhibiting electronic properties of carbon and redox tunability of transition metal is essential. Two-dimensional nanostructures of noble metals also enhance performance by preventing agglomeration and exposing active sites. Pd nanomeshes revealed superior activity (power density ~220.23 mW cm⁻²) than Pd- and Pt nanoparticles due to hole inner-edge reconstruction, exposure of Pd(111), (200), and (311) facets, nanoporous structure and stability in alkaline media [73]. Alloying noble-metals with TMs further optimizes intermediate adsorption energies via ensemble and ligand effects [65, 74, 75], as seen in Sn–Co/RuO₂ trimetallic oxides, where Sn doping regulated the electronic environment of Ru and Co (Figure 3e–f) and optimized *OOH/*OH adsorption/desorption, achieving ultralong cycle life (3312 h at 5 mA cm⁻²) [61].

Likewise, a CoCuFeAgRu high entropy alloy (HEA) anchored in N-doped carbon nanosheets boosted RZAB performance (power density ~136.53 mW cm⁻²), benefiting from Ag-Ru synergistic effects and tailored electronic structure [76]. HEAs are promising due to their disordered local arrangements, unique and tunable electronic structure, which can break the scaling relationships and stabilize intermediate adsorption energies during electrocatalysis [77]. Theoretical screening of alloy composition is required to ensure oxygen bifunctional activity, while the activity can be further improved via conductive supports (e.g., carbon, TM, MXenes), modulating charge distribution and metal-support interactions. Single-atom catalysts effectively reduce the amount of noble metal without compromising the activity due to enhanced atomic utilization. Ru single atoms on MnO₂ nanorods (Ru SAs@MnO₂) shifted the d-band center negatively (-0.553 eV) compared to MnO₂ (−0.475 eV) (Figure 3g–h), increasing charge density at Mn sites to promote ORR while Ru enhanced OER activity [62]. Future directions must emphasize on dual single-atomic sites to enhance bifunctional activity and simplified synthesis procedures in view of sustainability.

Non-noble metal-based catalysts: Despite the advancements in noble-metal catalysts, sustainability concerns have prompted a shift towards TM-based catalysts (e.g., oxides, sulfides, phosphides, borides) [78-82] for low-cost RZABs with reduced environmental footprint. Their bifunctional activity arises from vacant TM d-orbitals, modulated chemical binding with reactive oxygen intermediates [83]. Strategies such as active site optimization, surface functionalization, defect engineering, and morphology control have been pivotal in enhancing their catalytic performance [84-86]. Integrating multiple metal sites induces synergistic effects, creates bifunctional active sites, and tunes electronic structures, enabling reduced voltage gaps (< 0.8 V), surpassing the benchmark IrO₂ + Pt/C catalyst [87, 88]. A Cu–N–C catalyst functionalized with Na-containing groups (CuNa-CF) achieved over 5000 h of battery life (1 mA cm⁻²) due to reduced carbon corrosion, enhanced conductivity through unique electronic structure between Cu-N₄ and –COONa (Figure 4a) and accelerated O₂ kinetics via strong anchoring of OOH* at Cu-N₄-COONa depending on Bader charge transfer (Figure 4b) [89]. Beyond oxides, hydroxides, and chalcogenides, TM phosphides and borides reveal competent bifunctional activity. Electronegative phosphorus shifts the TM d-band center close to the Fermi level [93], increases e⁻ delocalization to induce more positive charges on TM [94], thereby enhancing redox activity. Covalent M-P bonds improve stability [95], while surface oxyhydroxides (M-OOH) and inorganic phosphates reduce OER overpotentials and improve wettability [79, 96]. Fe single-atom catalyst on hierarchical N, P co-doped carbon nanofibers with asymmetric Fe-N₃P₁-C sites exhibited enhanced ORR/OER performance due to increased Bader charge (7.237e) and thus increased charge density at Fe due to the presence of P atoms (Figure 4c) [90]. Further weak *O and *OH adsorption over Fe-N₃P₁-C enhanced ORR while lower ΔG (0.89 eV) stabilized *OOH and reduced energy barrier for OER (Figure 4d), respectively [90]. However, monometallic phosphides remain less active than noble metals, necessitating further engineering through doping, defect creation, and heterostructure design [97]. Multi-metallic phosphides demonstrate enhanced performance due to synergistic effects and altered electronic structures [95]. Further, preparing a composite with sulfides, carbon materials, etc., boosts bifunctional activity via enhanced charge-transfer properties [98, 99]. A catalyst derived from organophosphorus complex (1,1-bis(diphenylphosphine)ferrocene (DPPF)) underwent in situ conversion to form Fe-NiCoP nanoparticles confined in heteroatom-doped carbon matrix (NPC), boosting RZAB performance through strong Fe–NiCoP–NPC interactions [100]. Challenges such as surface oxidation and phase separation in multi-metallic phosphides persist [101, 102], highlighting the need for conductive carbon coatings and optimized synthetic routes to prepare stable single-phase materials [103]. TM borides are also promising for bifunctional oxygen catalysis, though their practical use is limited by tedious synthesis involving toxic gaseous precursors and expensive equipment, as well as susceptibility to oxidation [104, 105]. Simplified methods like sonochemical reduction have yielded stable Co₂B catalysts with minimal surface oxidation and particle agglomeration [85]. Amorphous CO₂B revealed high bifunctional activity via B-2p and Co-3d orbital hybridization and B-O species mediated intermediate adsorption for high-performance RZABs (power density ~500 mW cm⁻²) powering electrochemical ammonia synthesis for 2 h (NH₃ yield: 1.048 mg h⁻¹ mg_cat.⁻¹, Figure 4e) [85]. Doping secondary metals further enhances OER and reduces charging overpotentials by decreasing the Bader charges in metal and boron atoms, promoting surface reconstruction and formation of amorphous oxyhydroxide layers (Figure 4f) [91]. Major improvements in the bifunctional activity of TM phosphides/borides require dopant and defect engineering, environment-friendly and controlled synthesis routes to tune morphology and boost mass/electron transport. Single-atom catalysts offer significant opportunities, especially with dual-metal sites. A dual single-atom catalyst (FeCo-SAs) on N-doped graphene nanosheets achieved a cycle life of over 550 h at 10 mA cm⁻². The metal synergistic effect transformed Fe-N₄ to a more favorable and asymmetric N₃–Fe–Co–N₄ configuration during pyrolysis (Figure 4g), enhancing adsorption/desorption of ORR/OER intermediates [92]. However, low metal loadings are witnessed for single-atom catalysts, obliging efficient synthesis methods. A plasma-bombing synthesis [106] could achieve high loading of single atoms (2.5 wt%) in a Co single-atom catalyst anchored on N-doped carbon, enabling high ORR and thus discharge performance. Non-noble metal catalysts still require substantial improvements to surpass noble-metal benchmarks. Beyond experimental advancements, theoretical modeling remains crucial for unraveling mechanisms, optimizing catalyst design, and identifying active sites for next-generation bifunctional catalysts.

Carbon-based catalysts: Carbon-based catalysts have emerged as promising bifunctional oxygen catalysts due to high surface area, tunable activity, and scalable synthesis, including green routes utilizing biomass- or waste-derived precursors [107]. However, sluggish oxygen redox kinetics and poor chemical stability at high positive potentials remain challenging.

Heteroatom doping is widely employed to regulate the electronic structure and enhance both catalytic activity and stability [108]. While single-heteroatom doping shows limited bifunctionality, co-doping stabilizes oxygen intermediates via cooperative interactions between p-electrons of heteroatoms. Pyridinic N–B pair doped carbon microspheres (NB-CMS) with hierarchical micropore-mesopore structure achieved ORR and OER bifunctional activity with B@PyN configuration, as evidenced by their position on volcano plots correlating limiting potentials (U_L) with ΔG_⁎OH and ΔG_⁎O − ΔG_⁎OH, respectively (Figure 5a–b) [109]. Incorporating e⁻ donating functional groups further mitigates carbon corrosion by reducing the positive charge density [112]. Functional group modification also influences dopant distribution and accelerates oxygen kinetics. In parallel, defect engineering enhances intrinsic catalytic activity by altering the atomic and electronic structure of carbon through vacancy, topological, and grain boundary defects [113]. Hierarchical porous carbons, integrating micropores, mesopores, and macropores, demonstrate enhanced activity and stability in RZABs, where in micropores increase surface area, while meso- and macropores enhance reactant accessibility and mass transport [114]. Interfacial effects further amplify local electric fields and generate active sites. Unlike carbon materials with limited meso- and macropores, N,S co-doped carbon with a hierarchically porous structure (HPNSC) exhibits abundant micro-, meso- and macropores with larger pore volume (1.05 cm³ g⁻¹) and surface area (1261 m² g⁻¹), with enhanced electrochemically active surface area (ECSA ~ 429 m² g⁻¹) relative to non-hierarchical NSC (Figure 5c). The larger meso- and macropores in HPNSC expose more active N,S sites to O₂, highlighting the role of porous architecture in improving both ECSA and intrinsic activity beyond simple mass transport [110]. But still, achieving benchmark performance will require the development of cost-effective synthesis methods, rational bifunctional site engineering, and strategies to enhance carbon durability by mitigating corrosion and electrolyte carbonation. Heteroatom co-doping strategy, defect and hierarchical pore engineering are the major areas to focus on in the future, along with the development of corrosion-resistant coatings to improve performance and durability in RZABs. Besides, biomass-derived synthesis procedures are critical to improve the sustainability of carbon-catalysts. Metal-carbon composites have enlightened oxygen bifunctional electrocatalysis by integrating active metal sites with tunable carbon structures. While carbon materials exhibit high ORR activity, their limited OER performance needs incorporation of metal sites to induce bifunctionality [55, 115]. Optimizing metal loading on carbon substrates is critical to minimize charge-discharge overpotentials. Heteroatom doping, particularly with nitrogen, improves intermediate adsorption and catalytic activity. Metal-containing N-doped carbon (M-NC) catalysts benefit from strong metal–metal, metal–carbon, and metal–heteroatom interactions, contributing to superior ORR-OER activity [116]. But alternate synthesis methodologies beyond conventional pyrolysis like surfactant-assisted approaches [117], use of low-nucleation rate precursors and controlled electrospinning, must be adopted to minimize agglomeration [118, 119]. Recently, a quasi-phthalocyanine conjugated 2D covalent organic polymer (COP_BTC-Co) comprising CoN₄₊₄ sites was synthesized through a pyrolysis-free strategy. Additional N₄ atoms could alter the antibonding Co orbitals, weakening oxygen adsorption, thereby enhancing bifunctional activity and battery performance (100 h, 10 mA cm⁻²) [120]. Further, heteroatom co-doped metal-carbon composites can enhance cathode kinetics in RZABs, for example, trace single-atom Fe anchored on N-, S-codoped carbon (Fe-PNSC) could achieve a high-power density (122.2 mW cm⁻²) and prolonged cycle life up to 300 h at 5 mA cm⁻². This is attributed to the honeycomb-like porous structure (0.71 cm³ g⁻¹), large surface area (957.69 m² g⁻¹), single atom Fe-sites, and heteroatom co-doping (4.66 at.% N and 1.9 at.% S), providing modulated charge distribution and multiple mass transfer channels [121]. Though metal-oxide carbon composites exhibit bifunctional activity, long-term durability can be compromised by metal aggregation. Protective coatings help stabilize these composites during cycling and improve catalytic performance [122, 123]. Metal chalcogenides, phosphides, hydroxides, etc., improve bifunctional activity through enhanced conductivity, strong interactions, and facilitated electron transfer at metal–carbon interface. A graphdiyne-induced CoN/CoS₂ heterostructure in N-doped carbon interface (CoN/CoS₂/NC) perceived facilitated e⁻ transport, reduced carbon corrosion, and enhanced RZAB performance (voltage gap ~1.1 V at 100 mA cm⁻²) [111]. This is attributed to π–e⁻-rich alkyne interactions, pore engineering, and fine-tuned heterojunction (Figure 5d–e) with higher e⁻ density near the Fermi level due to strong CoN–CoS₂ interfacial interaction [111]. Similarly, a high entropy alloy (FeCoNiCuMg) carbon nanocomposite with a hierarchical porous structure comprising micro-, meso-, and macropores enhanced the metal active sites utilization [124]. Nonetheless, further research necessitate optimizing metal loading and ensuring their uniform dispersion during catalyst synthesis. Interface engineering through hybrid coordination environments (e.g., metal–heteroatom–carbon interactions, in situ formation of active sites) can drastically boost the cathode activity. Controlled synthesis procedures should also be looked after to avoid metal aggregation, enabling the practical use of metal–carbon composites. However, the complexity of metal-carbon composites results in poor fundamental understanding of bifunctional sites, obliging advanced in situ techniques to elucidate mechanisms. Molecular catalysts show promise, owing to their definite structure, which doesn't require complex techniques to understand the intrinsic activity [125], nonetheless, tuning their activity is challenging. Electrochemical performance of a Co-porphyrin-based catalyst (Co (III) meso-tetra (N-methyl-4-pyridyl) porphyrine) was improved by supramolecular assembly with chemically converted graphene through synergistic (electrostatic/π–π) interactions [126]. But again, molecular catalysts can easily degrade during ORR, due to the presence of oxygen free radicals [127]. Thus, stabilizing molecular catalysts under such environments while screening their structural evolution is crucial.

Table S1 and Figure 5f summarize different catalysts with cycling performance exceeding 100 h. A mixture of benchmark catalysts for ORR and OER (Pt/C + IrO₂) reveals inferior cycling performance ( < 100 h), where alloying with transition metals holds immense potential. Doping RuO₂ with Mn prolonged the cycle life up to 2500 h (10 mA cm⁻²) [31]. In addition, bimetallic composites (Sn–Co/RuO₂: 3312 h @ 5 mA cm⁻²) [61], single atom catalysts (Ru_SA-NiFe LDH: 2400 h @ 10 mA cm⁻²) [60] and HEAs (CoCuFeAgRu: 1650 h @ 20 mA cm⁻²) [76] show utmost potential to improve the performance of noble-metal catalysts. In contrast, TM-based catalysts show inferior performance than noble-metal counterparts, with TM phosphides being most promising, for example, NiCoP@CoNi-LDH/SSM could operate over 1505 h at 5 mA cm⁻² [128]. Tailoring bifunctionality in TM phosphides via doping, alloying, heterostructuring, and morphology tuning as well as employing bimetallic and single-atom configurations, will boost battery efficiency. Metal-carbon composites provide combined benefits from metal centers and carbon. A bimetallic CuNa-CF catalyst ensured a high cycling performance of 5000 h (1 mA cm⁻²) and 950 h (50 mA cm⁻²) via C₂-Cu-N₄ sites (provides stability during OER) and Na (boosts activity of Cu-N₄ center) [89]. Further, TM phosphide composite with heteroatom-doped carbon shows massive potential, for instance, a Co/Co₂P@NCNT catalyst based Zn-air battery enabled 1080 h of operation at 5 mA cm⁻². This summary of the most promising catalysts depicts that noble-metal-based catalysts hold the highest potential to achieve practical performance requiring reduced noble-metal content while retaining the maximum utilization. While numerous efficient, robust, and cost-effective oxygen bifunctional catalysts have been reported, they often fall short of meeting practical performance and sustainability demands. Follow-up efforts combining electrode design and engineering using relatively green synthesis procedures, electrolyte optimization, and mechanistic understanding are essential to realize practical RZABs. Moreover, air cathodes must be evaluated under practical conditions, i.e., high current densities and operating temperatures, to fill the gap between laboratory research and commercial applications.

3.2.1 Air Cathode

Air cathode degradation arises from carbonate precipitation, electrolyte flooding, cathode erosion, and GDL blockage (Figure 6a), ultimately shortening lifespan and even battery failure, as confirmed by manufacturing companies (Yardney Electric Corporation [131], Electric Fuel Limited [132]) and mathematical modeling [133]. The main culprit is CO₂ interference from the air, inducing carbonate precipitation and blocking O₂ diffusion [134]. CO₂ removal strategies like adsorption, absorption, membrane separation, and gas cleansing have been explored, though bulky equipment hinders practical use [135]. An electrically rechargeable flow-type Zn-air battery was scaled up with a CO₂ adsorption unit, revealing enhanced durability (760 h), though battery failure was attributed to electrolyte flooding rather than carbonate blockage [136]. Integrating CO₂-adsorbing features directly into cathodes offers a more scalable solution. Additionally, carbon corrosion at high oxidation potentials and O₂ bubble accumulation during charging must be addressed. Improvements in electrode design such as (a) polymer coatings to reduce corrosion, (b) engineered GDLs for better electron/mass transport and manage electrolyte flooding, and (c) defect engineering to prevent catalyst degradation, have shown promise to overcome durability issues.

3.2.2 Anode and Electrolyte

At anode, Zn dendrite formation (causing short circuit), parasitic reactions with electrolyte, passivation, and corrosion deteriorate performance and durability, leading to battery failure (Figure 6b). Suppressing dendrites and minimizing side reactions are crucial to extending battery life [137]. Uniform Zn deposition through surface modification and electrolyte engineering helps control dendritic growth by modifying the surface electric field and boosting zincate ion transport. Protective organic/inorganic coatings with high Zn affinity reduce desolvation energy and nucleation barriers, promote uniform Zn deposition and preventing direct contact between Zn and electrolyte, thereby enhancing battery lifespan and safety. This is attributed to the strong

coordination of organic groups with Zn (through H-bonding) to prevent direct contact between the anode and active H₂O via replacement of H₂O in Zn solvation sheath [138]. While inorganic coatings like TiO₂, graphene, etc., mitigate anodic issues via layered adsorption effect and bonding with H₂O molecules, their high cost and complexity advance low-cost organic coatings [139-141]. Metal-organic frameworks (MOFs) having high electronegative functional groups can easily form H-bonds with H₂O in [Zn(H₂O)₆]²⁺ [142], and polymer coatings (e.g., polyvinyl alcohol [143], polyacrylonitrile [144]) also show efficacy but require optimized conductivity, thickness, and wettability. Conductive carbon coatings such as graphene oxide aid e⁻ transfer and reduce Zn dissolution, minimizing anode passivation [140]. A chemical buffer layer (ZnO nanorods dispersed in graphene oxide [GO]) coated Zn anode (CBL@Zn) improved Zn reversibility and enabled long-term cycling of 450 h, attributed to ZnO nanorods limiting nuclei overgrowth and GO regulating precipitation-dissolution behavior of insulating ZnO through adsorption and confinement of zincate ions (Figure 6c–d) [129]. Anion-exchange ionomers can recover Zn during electrodeposition from oxidized Zn discharge products, enhancing durability [145]. Inorganic corrosion inhibitors (e.g., Bi, Pb, Ni, TiO₂, Bi₂O₃, Al₂O₃, Si(OH)₄) suppress HER by increasing its overpotential, but concerns over toxicity, cost, and environmental drive interest in eco-friendly organic inhibitors like polyethylene glycol and polyacrylonitrile, which bind Zn via hydrophilic groups to inhibit side reactions [146, 147]. In the future, supramolecular coatings with biological Zn-binding sites [148], water-in-salt electrolytes [149], and self-healing polymer coatings [150] should be largely focused to promote desolvation and improve Zn reversibility. Electrolyte additives also influence dendritic growth and unwanted reactions. Organic additives (e.g., cetyltrimethylammonium bromide, sodium dodecyl sulfate, polyethylene glycol, thiourea) influence anode surface morphology to reduce dendrites, HER, and Zn corrosion [130, 151-153]. Inorganic additives (e.g., Ni, Pb, CuCl₂, CuO, Zn(ClO₄)₂) promote homogeneous Zn deposition by metal ions with higher reduction potentials than Zn, which can undergo reduction before Zn and tune the electron density at anode [154]. Anti-corrosion surfactants/additives like sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, etc., inhibit ZnO formation and anode passivation [155, 156]. A cationic surfactant (trimethyloctadecylammonium chloride (STAC)) efficiently suppressed dendrite growth via adsorption of ammonium cations onto uneven Zn surface via electrostatic interactions, promoting lateral growth to form a smooth Zn layer ( ~17 μm, Figure 6e) [130]. In summary, the engineering of anode, cathode, and electrolyte altogether, can improve the durability of RZABs for practical applications (Figure 6f).

3.2.3 Separator

Separator stability is critical in RZABs, as dendritic growth can puncture the separator, causing short circuits and failure. Degradation is also linked to the crossover of anode discharge products and deposition at the cathode [157]. Commercial separators, like Celgard, offer mechanical strength but feature large pores that allow zincate crossover [158]. To optimize separators, improvements in ion selectivity, chemical stability, and conductivity are required. Commercial separators often exhibit hydrophobic properties, limiting ionic conductivity. Functionalization of separators, such as sulfonation of microporous Celgard 2320 separator, improved ionic conductivity by 132% (3.52 × 10⁻² S cm⁻¹) [159]. An electrospun nanofiber mat-reinforced composite (ERC) membrane prepared by polyvinyl alcohol-impregnated electrospun polyetherimide nanofiber mats prevents zincate crossover without compromising OH⁻ conduction (Figure 7a), as visualized using thymol blue indicator (Figure 7b) [160]. Selective anion exchange membranes (AEMs) improve ionic selectivity by permitting OH⁻ transport while retarding Zn²⁺ crossover, mitigating ZnO precipitation at cathode. AEMs grafted with N,N-diallylpiperidinium chloride (DAPCl) monomers on modified PPO improved conductivity, alkaline stability, and reduced zincate crossover. A trade-off between power density and zincate crossover is realized, signifying that despite similar power densities of PPO-6CH₂-3x and PPO-6CH₂-2x AEMs, PPO-6CH₂-2x are more durable due to reduced crossover [162].

Composite membranes made from affordable biopolymers offer environmental friendliness, stability, and good electrolyte absorption. A quaternized polyvinyl alcohol (QPVA)-chitosan (CS) separator embedded with molybdenum disulfide (MoS₂) revealed (a) high conductivity (87.3 mS cm⁻¹) via Grotthus and vehicular mechanisms and surface hopping, enabling OH⁻ transfer and (b) prolonged lifespan over 270 h due to enhanced stability via electrostatic interactions between polycationic polymer and negatively charged MoS₂ and reduced zincate crossover, resulting in less ZnO deposition at cathode (Figure 7c) [161]. Despite progress (Figure 7d), separator development for RZABs lags behind, presenting significant room for advancement.

3.2.4 Cell-Configuration

RZABs face durability issues due to anode, cathode, separator, and electrolyte degradation, with the accumulation of solid ZnO discharge products being a key challenge. These discharge products must be removed or reduced back to Zn metal during prolonged cycling. Electric Fuel Limited introduced separator bags (Figure 8a) to recover metal oxides during anode replacement but failed to prevent Zn passivation or reduce electrolyte resistance [165]. A flow-type system (Figure 8b) with continuous electrolyte circulation could address these issues by removing H₂ bubbles and ZnO, but the added complexity and manufacturing costs of electrolyte tanks, circulation pumps hamper practical deployment [166]. Additionally, flowing electrolytes also reduce O₂ transport and battery durability. Alternative strategies include solid or semi-solid electrolytes (Figure 8c) like alkaline anion exchange membranes, gel polymers, hydrogels, etc. [167-169]. AEMs impede Zn²⁺ crossover and self-corrosion; however, their limited hydrophilicity causes membrane drying, and quaternary ammonium groups degrade in alkaline environments [170]. Solid/semi-solid RZABs alleviate the leakage risks related to liquid electrolytes due to their enhanced electrochemical stability and mechanical strength, as well as suppress dendritic growth via uniform Zn deposition due to stable ion transfer. Gel polymer electrolytes offer high hydrophilicity and improved ORR/OER kinetics but suffer degradation due to excessive water uptake, rapid consumption of OH⁻ ions, reduced ionic conductivity, and quick evaporation of surface water/liquid phase (alkali climbing [125]). Further improvements in ionic conductivity, water retention, and management remain crucial for real-time application. Cross-linking polymers like polyethylene oxide, polyvinyl alcohol, etc. [171] enhance mechanical stability, while biopolymers (e.g., cellulose [172], chitosan [161]) provide additional benefits of biodegradability, non-toxicity, ease of synthesis, and cost-effectiveness.

In summary, RZABs face significant challenges due to durability issues across all battery components. This perspective emphasizes how to address these issues through targeted component engineering and optimizing cell configuration. Cathode improvements involve the integration of CO₂ adsorption sites, advanced electrode designs, and modified GDLs, though scalability remains a concern due to the complexity. Anode can be stabilized through surface modifications, protective coatings, and eco-friendly electrolyte additives. Electrolyte additives can aid in uniform Zn deposition, but issues like flooding and degradation persist. Separators are critical in preventing dendrite penetration and zincate crossover, with functional membranes and biopolymer composites offering improvements, yet the solutions for commercial separators lag behind. Most importantly, the solid ZnO buildup and H₂ gas accumulation further hinder the battery's reversibility and safety. Flow-type systems and semi-solid/solid electrolytes show promise but introduce new challenges like complexity, high cost, and membrane instability under alkaline conditions. Despite these hurdles, continued material and design innovations are bringing RZABs closer to commercial viability.