Hydrophobization Engineering of the Air-Cathode Catalyst for Improved Oxygen Diffusion towards Efficient Zinc-Air Batteries

Self-Charging Aqueous Zn//COF Battery with UltraHigh Self-Charging Efficiency and Rate

Self-charging zinc batteries that combine energy harvesting technology with batteries are candidates for reliable self-charging power systems. However, the lack of rational materials design results in unsatisfactory self-charging performance. Here, a covalent organic framework containing pyrene-4,5,9,10-tetraone groups (COF-PTO) is reported as a cathode material for aqueous self-charging zinc batteries. The ordered channel structure of the COF-PTO provides excellent capacity retention of 98% after 18 000 cycles at 10 A g-1 and ultra-fast ion transfer. To visually assess the self-charging performance, two parameters, namely self-charging efficiency (self-charging discharge capacity/galvanostatic discharge capacity, η) and average self-charging rate (total discharge capacity after cyclic self-charging/total cyclic self-charging time, ν), are proposed for performance evaluation. COF-PTO achieves an impressive η of 96.9% and an ν of 30 mAh g-1 self-charge capacity per hour in 100 self-charging cycles, surpassing the previous reports. Mechanism studies reveal the co-insertion of Zn2+ and H+ double ions in COF-PTO of self-charging zinc batteries. In addition, the C═N and C═O (on the benzene) in COF-PTO are ortho structures to each other, which can easily form metal heterocycles with Zn ions, thereby driving the forward progress of the self-charging reaction and enhancing the self-charging performance.

Cold-plasma activation converting conductive agent in spent Li-ion batteries to bifunctional oxygen reduction/evolution electrocatalyst for zinc-air batteries

Considerable amount of high-value transition metals components can be recycled in spent ternary lithium-ion batteries. In this study, we utilized the conductive agent carbon black, obtained from the leaching waste resulting from the chemical recovery of spent lithium-nickel-manganese-cobalt (NCM) oxide cathode materials. This process allows us to create valuable bifunctional catalysts for the oxygen reduction reaction and oxygen evolution reaction (ORR/OER), facilitated by a facile cold plasma activation method, as a part of lithium batteries circular economy. The activated conductive agent (RCA-30) exhibited an ORR half-wave potential of 0.74 V (vs. RHE) in 0.1 mol/L KOH solution, and an OER overpotential of 360 mV at 10 mA cm-2 in 1 mol/L KOH electrolyte, owing to nitrogen doping of carbon black and activation of surface metal oxides. The complete zinc-air batteries incorporating the activated catalysts at the cathode exhibited an open circuit potential of up to 1.48 V and sustained cycling for 100 h at a current density of 5 mA cm-2. Additionally, the activated catalysts contributed to a power density of 92 mW cm-2 and a full discharge capacity of 640 mAh/g.

Enhanced Oxygen Accumulation for a Hydrophobic Cathode in Lean-Oxygen Seawater Batteries

The unsatisfactory oxygen reduction reaction (ORR) kinetics caused by the inherent lean-oxygen marine environment brings low power density for metal-dissolved oxygen seawater batteries (SWBs). In this study, we propose a seawater/electrode interfacial engineering strategy by constructing a hydrophobic coating to realize enhanced mass transfer of dissolved oxygen for the fully immersed cathode of SWBs. Accumulation of dissolved oxygen from seawater to the catalyst is particularly beneficial for improving the ORR performance under lean-oxygen conditions. As a result, SWB assembled with a hydrophobic cathode achieved a power density of up to 2.32 mW cm-2 and sustained discharge at 1.3 V for 250 h. Remarkably, even in environments with an oxygen concentration of 4 mg L-1, it can operate at a voltage approximately 100 mV higher than that of an unmodified SWB. The introduction of a hydrophobic interface enhances the discharge voltage and power of SWBs by improving interfacial oxygen mass transfer, providing new insights into improving the underwater ORR performance for practical SWBs.

Ultrathin zigzag-surface copper nanowire assembled hierarchical microspheres to enhance oxygen reduction catalysis

Ultrathin catalysts predominantly expose surface active atoms to deliver promising applications in oxygen reduction reactions (ORRs). However, they are commonly synthesized at high reaction temperatures, with tedious chemical routes involved. Herein, we report a low temperature (273 K) electric field driven route to synthesize zigzag-surface ultrathin copper nanowires. Interestingly, the ultrathin copper nanowires assemble into three-dimensional microspheres, which exhibit hydrophobic-aerophilic features, eventually resulting in good ORR activities. The aerophilicity and hydrophobicity of copper nanowires are related to their Cu2O active sites and hierarchical protuberances, respectively. Our findings open a new door to grow ultrathin catalysts for new energy storage systems.

High-Power-Density Rechargeable Hybrid Alkali/Acid Zn-Air Battery Performance Through Value-Added Conversion Charging

Rechargeable Zn-air batteries (ZABs) are considered highly competitive technologies for meeting the energy demands of the next generation, whether for energy storage or portable power. However, their practical application is hindered by critical challenges such as low voltage, CO2 poisoning at the cathode, low power density, and poor charging efficiency Herein, a rechargeable hybrid alkali/acid Zn-air battery (h-RZAB) that effectively separates the discharge process in an acidic environment from the charging process in an alkaline environment, utilizing oxygen reduction reaction (ORR) and glycerol oxidation reaction (GOR) respectively is reported. Compared to previously reported ZABs, this proof-of-concept device demonstrates impressive performance, exhibiting a high power density of 562.7 mW cm-2 and a high operating voltage during discharging. Moreover, the battery requires a significantly reduced charging voltage due to the concurrent utilization of biomass-derived glycerol, resulting in practical and cost-effective advantages. The decoupled system offers great flexibility for intermittently generated renewable power sources and presents cost advantages over traditional ZABs. As a result, this technology holds significant promise in opening avenues for the future development of renewable energy-compatible electrochemical devices.

Hydrophobicity Promoted Efficient Hydroxyl Radical Generation in Visible-Light-Driven Photocatalytic Oxidation

Hydroxyl radical (•OH) with strong oxidation capability is one of the most important reactive oxygen species. The generation of •OH from superoxide radicals (•O2 -) is an important process in visible-light-driven photocatalysis, but the conversion generally suffers from slow reaction kinetics. Here, a hydrophobicity promoted efficient •OH generation in a visible-light-driven semiconductor-mediated photodegradation reaction is reported. Hydrophobic TiO2 that is synthesized by modifying the TiO2 surface with a thin polydimethylsiloxane (PDMS) layer and rhodamine B (RhB) are used as model semiconductors and dye molecules, respectively. The surface hydrophobicity resulted in the formation of a solid-liquid-air triphase interface microenvironment, which increased the local concentration of O2. In the meanwhile, the saturated adsorption quantity of RhB on hydrophobic TiO2 is improved by five-fold than that on untreated TiO2. These advantages increased the density of the conduction band photoelectrons and •O2 - generation, and stimulated the conversion of •O2 - to •OH. This consequently not only increased the kinetics of the photocatalytic reaction by an order of magnitude, but also altered the oxidation route from conventional decolorization to mineralization. This study highlights the importance of surface wettability modulation in boosting •OH generation in visible-light-driven photocatalysis.

Surface atom knockout for the active site exposure of alloy catalyst

The fine regulation of catalysts by the atomic-level removal of inactive atoms can promote the active site exposure for performance enhancement, whereas suffering from the difficulty in controllably removing atoms using current micro/nano-scale material fabrication technologies. Here, we developed a surface atom knockout method to promote the active site exposure in an alloy catalyst. Taking Cu3Pd alloy as an example, it refers to assemble a battery using Cu3Pd and Zn as cathode and anode, the charge process of which proceeds at about 1.1 V, equal to the theoretical potential difference between Cu2+/Cu and Zn2+/Zn, suggesting the electricity-driven dissolution of Cu atoms. The precise knockout of Cu atoms is confirmed by the linear relationship between the amount of the removed Cu atoms and the battery cumulative specific capacity, which is attributed to the inherent atom-electron-capacity correspondence. We observed the surface atom knockout process at different stages and studied the evolution of the chemical environment. The alloy catalyst achieves a higher current density for oxygen reduction reaction compared to the original alloy and Pt/C. This work provides an atomic fabrication method for material synthesis and regulation toward the wide applications in catalysis, energy, and others.

An Overview and Future Perspectives of Rechargeable Flexible Zn-Air Batteries

Environmental friendliness and low-cost zinc-air batteries for flexible rechargeable applications have great potential in the field of flexible electronics and smart wearables owing to high energy density and long service life. However, the current technology of flexible rechargeable zinc-air batteries to meet the commercialization needs still facing enormous challenges due to the poor adaptability of each flexible component of the zinc-air batteries. This review focused on the latest progress over the past 5 years in designing and fabricating flexible self-standing air electrodes, flexible electrolytes and zinc electrodes of flexible Zn-air batteries, meanwhile the basic working principle of each component of flexible rechargeable zinc-air batteries and battery structures optimization are also described. Finally, challenges and prospects for the future development of flexible rechargeable zinc-air batteries are discussed. This work is intended to provide insights and general guidance for future exploration of the design and fabrication on high-performance flexible rechargeable zinc-air batteries.

Modulation of Phase Transition in Cobalt Selenide with Simultaneous Construction of Heterojunctions for Highly-Efficient Oxygen Electrocatalysis in Zinc-Air Battery

Phase transformation of cobalt selenide (CoSe2 ) can effectively modulate its intrinsic electrocatalytic activity. However, enhancing electroconductivity and catalytic activity/stability of CoSe2 still remains challenging. Heterostructure engineering may be feasible to optimize interfacial properties to promote the kinetics of oxygen electrocatalysis on a CoSe2 -based catalyst. Herein, a heterostructure consisting of CoSe2 and cobalt nitride (CoN) embedded in a hollow carbon cage is designed via a simultaneous phase/interface engineering strategy. Notably, the phase transition of orthorhombic-CoSe2 to cubic-CoSe2 (c-CoSe2 ) accompanied by in situ CoN formation is realized to build the c-CoSe2 /CoN heterointerface, which exhibits excellent/highly stable activities for oxygen reduction/evolution reactions (ORR/OER). Notably, heterostructure can modulate the local coordination environment and increase Co-Se/N bond lengths. Theoretical calculations show that Co-site (c-CoSe2 ) with an electronic state near Fermi energy level is the main active site for ORR/OER.Energetical tailoring of the d-orbital electronic structure of the Co atom of c-CoSe2 in heterostructure by in situ CoN incorporation lowers thermodynamic barriers for ORR/OER. Attractively, a zinc-air battery with a c-CoSe2 -CoN cathode displays excellent cycling stability (250 h) and charge/discharge voltage loss (0.953/0.96 V). It highlights that heterointerface engineering provides an option for modulating the bifunctional activity of metal selenides with controlled phase transformation.

Mesoporous Carbon for High-Performance Near-Neutral Zinc-Air Batteries

Near-neutral zinc-air batteries (ZABs) have garnered significant research interest due to their high energy density, exceptional electrochemical reversibility, and adaptability to ambient air. However, these batteries suffer from substantial electrochemical polarization, low energy efficiency, and poor rate performance. In this study, a mesoporous carbon (meso-C) with a high specific surface area (1081 m2 g-1 ) and abundant porous structure for the cathode of near-neutral ZABs using a scalable synthesis method is prepared. The meso-C-based cathode is endowed with stable hydrophobicity and abundant electrochemical active sites, which considerably improve the energy efficiency, rate performance, and cycle life of the battery compare to commercial carbon black-based cathode when applied to near-neutral ZABs with 1 mol kg-1 (1 m) zinc acetate and 1 m zinc trifluoromethanesulfonate electrolytes. Additionally, the mesopores of meso-C facilitate the construction of better three-phase reaction interfaces and contribute to better electrochemical reversibility. The work presents a general and scalable approach for carbon materials in the cathode of near-neutral ZABs.

Ultrafine Pt Nanoparticles Anchored on 2D Metal-Organic Frameworks as Multifunctional Electrocatalysts for Water Electrolysis and Zinc-Air Batteries

Multifunctional electrocatalysts are crucial to cost-effective electrochemical energy conversion and storage systems requiring mutual enhancement of disparate reactions. Embedding noble metal nanoparticles in 2D metal-organic frameworks (MOFs) are proposed as an effective strategy, however, the hybrids usually suffer from poor electrochemical performance and electrical conductivity in operating conditions. Herein, ultrafine Pt nanoparticles strongly anchored on thiophenedicarboxylate acid based 2D Fe-MOF nanobelt arrays (Pt@Fe-MOF) are fabricated, allowing sufficient exposure of active sites with superior trifunctional electrocatalytic activity for hydrogen evolution, oxygen evolution, and oxygen reduction reactions. The interfacial Fe─O─Pt bonds can induce the charge redistribution of metal centers, leading to the optimization of adsorption energy for reaction intermediates, while the dispersibility of ultrafine Pt nanoparticles contributes to the high mass activity. When Pt@Fe-MOF is used as bifunctional catalysts for water-splitting, a low voltage of 1.65 V is required at 100 mA cm-2 with long-term stability for 20 h at temperatures (65 °C) relevant for industrial applications, outperforming commercial benchmarks. Furthermore, liquid Zn-air batteries with Pt@Fe-MOF in cathodes deliver high open-circuit voltages (1.397 V) and decent cycling stability, which motivates the fabrication of flexible quasisolid-state rechargeable Zn-air batteries with remarkable performance.

Hydrophobic and Homogeneous Conductive Carbon Matrix for High-Rate Non-Alkaline Zinc-Air Batteries

Non-alkaline zinc-air batteries (ZABs) that use reversible O2 /ZnO2 chemistry exhibit excellent stability and superior reversibility compared to conventional alkaline ZABs. Unlike alkaline ZABs, ZnO2 discharge products are generated on the surface of the air cathodes in non-alkaline ZABs, requiring more gas-liquid-solid three-phase reaction interfaces. However, the kinetics of reported ZABs based on carbon black (CB) is far from satisfactory due to the insufficient reaction areas. The rational structural design of the air cathode is an effective way to increase active surfaces to further enhance the performance of non-alkaline ZABs. In this study, multi-walled carbon nanotubes (MW-CNTs) with unique mesoporous structures and high pore volumes are selected to replace CB in the air cathode preparation. Due to the larger electrochemically active surface area, superior hydrophobicity, and uniform electroconductibility of MW-CNTs-based cathodes, primary ZABs exhibit high specific capacity (704 mAh gZn-1 ) with a Zn utilization ratio of 85.85% at 1.0 mA cm-2 , excellent discharge rate performance, and negligible self-discharge. Furthermore, rechargeable ZABs also demonstrate outstanding rate capability and excellent cycling stability at various current densities. This work provides a fundamental understanding of the criteria for the cathode design of non-alkaline ZABs, thus opening a new pathway for more sustainable ZABs.

Long-cycle Zn-air batteries at high depth of discharge enabled by a robust Zn|electrolyte interface

It is an urgent need to improve the depth of discharge (DOD) of Zn-air batteries (ZABs), considering that most reported ZABs with long cycle life are realized at low DOD (<1%). In this work, our solid-state ZABs achieved a long cycle life of more than 220 h at 3.2% DOD (the discharge capacity of 10 mA h cm-2 per cycle). Moreover, benefiting from excellent bifunctional oxygen electrocatalysts (Fe@BNC) and robust Zn|electrolyte interface, the ZABs displayed a long cycle life of 120 h even at high DOD of 23.4% and large discharge capacity of 72 mA h cm-2. Additionally, the impact of Zn|electrolyte interface on the cycle time at different DODs is analysed and discussed. The unstable interface exacerbated the dendrite growth and uneven deposition of Zn at high DOD, leading to the decay of the cycle life. The work gives insights into the mechanism of the effect of DOD on the cycle life of the batteries.

Highly-Exposed Co-CoO Derived from Nanosized ZIF-67 on N-Doped Porous Carbon Foam as Efficient Electrocatalyst for Zinc-Air Battery

Non-precious-metal based electrocatalysts with highly-exposed and well-dispersed active sites are crucially needed to achieve superior electrocatalytic performance for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) toward zinc-air battery (ZAB). Herein, Co-CoO heterostructures derived from nanosized ZIF-67 are densely-exposed and strongly-immobilized onto N-doped porous carbon foam (NPCF) through a self-sacrificial pyrolysis strategy. Benefited from the high exposure of Co-CoO heterostructures and the favorable mass and electron transfer ability of NPCF, the Co-CoO/NPCF electrocatalyst exhibits remarkable performance for both ORR (E1/2  = 0.843 V vs RHE) and OER (Ej = 10 mA cm-2  = 1.586 V vs RHE). Further application of Co-CoO/NPCF as the air-cathode in rechargeable ZAB achieves superior performance for liquid-state ZAB (214.1 mW cm-2 and 600 cycles) and flexible all-solid-state ZAB (93.1 mW cm-2 and 140 cycles). Results from DFT calculations demonstrate that the electronic metal-support interactions between Co-CoO and NPCF via abundant C-Nx sites is favorable for electronic structure modulation, accounting for the remarkable performance.

Recent advances of bifunctional catalysts for zinc air batteries with stability considerations: from selecting materials to reconstruction

With the growing depletion of traditional fossil energy resources and ongoing enhanced awareness of environmental protection, research on electrochemical energy storage techniques like zinc-air batteries is receiving close attention. A significant amount of work on bifunctional catalysts is devoted to improving OER and ORR reaction performance to pave the way for the commercialization of new batteries. Although most traditional energy storage systems perform very well, their durability in practical applications is receiving less attention, with issues such as carbon corrosion, reconstruction during the OER process, and degradation, which can seriously impact long-term use. To be able to design bifunctional materials in a bottom-up approach, a summary of different kinds of carbon materials and transition metal-based materials will be of assistance in selecting a suitable and highly active catalyst from the extensive existing non-precious materials database. Also, the modulation of current carbon materials, aimed at increasing defects and vacancies in carbon and electron distribution in metal-N-C is introduced to attain improved ORR performance of porous materials with fast mass and air transfer. Finally, the reconstruction of catalysts is introduced. The review concludes with comprehensive recommendations for obtaining high-performance and highly-durable catalysts.

Decoding the Mechanisms of Reversibility Loss in Rechargeable Zinc-Air Batteries

Attaining high reversibility of the electrodes and electrolyte is essential for the longevity of secondary batteries. Rechargeable zinc-air batteries (RZABs), however, encounter drastic irreversible changes in the zinc anodes and air cathodes during cycling. To uncover the mechanisms of reversibility loss in RZABs, we investigate the evolution of the zinc anode, alkaline electrolyte, and air electrode through experiments and first-principles calculations. Morphology diagrams of zinc anodes under versatile operating conditions reveal that the nanosized mossy zinc dominates the later cycling stage. Such anodic change is induced by the increased zincate concentration due to hydrogen evolution, which is catalyzed by the mossy structure and results in oxide passivation on electrodes and eventually leads to low true Coulombic efficiencies and short life spans of batteries. Inspired by these findings, we finally present a novel overcharge-cycling protocol to compensate for the Coulombic efficiency loss caused by hydrogen evolution and significantly extend the battery life.

Bioinspired Hydrophobicity Coupled with Single Fe-N4 Sites Promotes Oxygen Diffusion for Efficient Zinc-Air Batteries

The poor oxygen diffusion and sluggish oxygen reduction reaction (ORR) kinetics at multiphase interfaces in the cathode suppress the practical application of zinc-air batteries. Developing effective strategies to tackle the issue is of great significance for overcoming the performance bottleneck but remains challenging. Here, a multiscale hydrophobic surface is designed on the iron single-atom catalyst via a gas-phase fluorination-assisted method inspired by the structure of gas-trapping mastoids on lotus leaves. The hydrophobic Fe-FNC attains a higher peak power density of up to 226 mW cm^-2 , a long durability of up close to 140 h, and better cyclic durability of up to 300 cycles compared to the corresponding Pt/C-based Zn-air battery. Experiments and theoretical calculations indicate that the formed more triple-phase interfaces and exposed isolated Fe-N₄ sites are proposed as the governing factors in boosting electrocatalytic ORR activity and remarkable cycling durability for Zn-air batteries.

Developing Superior Hydrophobic 3D Hierarchical Electrocatalysts Embedding Abundant Catalytic Species for High Power Density Zn-Air Battery

It is essential but still challenging to design and construct inexpensive, highly active bifunctional oxygen electrocatalysts for the development of high power density zinc-air batteries (ZABs). Herein, a CoFe-S@3D-S-NCNT electrocatalyst with a 3D hierarchical structure of carbon nanotubes growing on leaf-like carbon microplates is designed and prepared through chemical vapour deposition pyrolysis of CoFe-MOF and subsequent hydrothermal sulfurization. Its 3D hierarchical structure shows excellent hydrophobicity, which facilitates the diffusion of oxygen and thus accelerates the oxygen reduction reaction (ORR) kinetic process. Alloying and sulfurization strategies obviously enrich the catalytic species in the catalyst, including cobalt or cobalt ferroalloy sulfides, their heterojunction, core-shell structure, and S, N-doped carbon, which simultaneously improve the ORR/OER catalytic activity with a small potential gap (ΔE = 0.71 V). Benefiting from these characteristics, the corresponding liquid ZABs show high peak power density (223 mW cm^-2 ), superior specific capacity (815 mA h g_Zn ^-1 ), and excellent stability at 5 mA cm^-2 for ≈900 h. The quasi-solid-state ZABs also exhibit a very high peak power density of 490 mW cm^-2 and an excellent voltage round-trip efficiency of more than 64%. This work highlights that simultaneous composition optimization and microstructure design of catalysts can effectively improve the performance of ZABs.

A Fluorinated Covalent Organic Framework with Accelerated Oxygen Transfer Nanochannels for High-Performance Zinc-Air Batteries

The establishment of abundant three-phase interfaces with accelerated mass transfer in air cathodes is highly desirable for the development of high-rate and long-cycling rechargeable zinc-air batteries (ZABs). Covalent organic frameworks (COFs) exhibit tailored nanopore structures, facilitating the rational tuning of their specific properties. Here, by finely tuning the fluorinated nanopores of a COF, a novel air cathode for rechargeable ZABs is unprecedentedly designed and synthesized. COF nanosheets are decorated with fluorinated alkyl chains, which shows high affinity to oxygen (O₂ ), in its nanopores (fluorinated COF). The fluorinated COF nanosheets are stacked into well-defined O₂ -transport channels, which are then assembled into aerophilic "nano-islands" on the hydrophilic FeNi layered-double-hydroxide (FeNi LDH) electrocatalyst surface. Therefore, the mass-transport "highway" for O₂ and water is segregated on the nanoscale, which significantly enlarges the area of three-phase boundaries and greatly promotes the mass-transfer therein. ZABs based on the COF-modified air cathode deliver a small charge/discharge voltage gap (0.64 V at 5 mA cm^-2 ), a peak power density (118 mW cm^-2 ), and a stable cyclability. This work provides a feasible approach for the design of the air cathodes for high-performance ZABs, and will expand the new application of COFs.

Tailoring the Electrochemical Deposition of Zn by Tuning the Viscosity of the Liquid Electrolyte

The issues during Zn deposition in rechargeable Zn-based batteries greatly hinder cycling stability. In this work, a simple and inexpensive approach to tailor the Zn electrodeposition is proposed by tuning the viscosity of the liquid electrolyte (LE). First, the growth mechanisms of Zn deposition under different electrolyte properties are investigated by numerical simulation, from which the bottom deposition tends to fuse with each other when there are more deposition sites, and the mass-transfer coefficient is lower, thus achieving uniform deposition. Besides, the whole process of Zn deposition in charging-discharging cycling is in situ observed by an optical microscope. It is found that the cause of the poor stability in the LE is due to the uneven Zn deposition, resulting in weak bonding between the deposition and the electrode surface, which is also the reason for the formation of dead Zn. In contrast, when an appropriate amount of the polymer is added to the LE to increase the viscosity, an appropriate overpotential can be created, generating more deposition sites. In addition, the viscosity reduces the mass-transfer coefficient, making the distance from the ion to the deposition sites the main controlling factor. The Zn ions are more inclined to move in the direction of electric field lines, which results in a uniform and dense deposition layer. Furthermore, the effectiveness of this method is demonstrated in a Zn-LiFePO₄ battery, from which the battery with the modified electrolyte condition still works properly even in the Zn utilization of 100% and shows a capacity retention rate (35%) of nearly twice that in the original LE condition (18%) after 10 cycles. This work provides a theoretical basis for Zn deposition and provides ideas for the future development of high-performance Zn-based batteries.

An Ultrastable Rechargeable Zinc-Air Battery Using a Janus Superwetting Air Electrode

The rechargeable zinc-air batteries (ZABs) are promising energy storage devices, but their performance is limited by the air electrode, coming from the contradictory wettability requirements of the air electrode at charging and discharging. Herein, to improve the mass transport and adapt to its different requirements when charging and discharging the ZABs, a Janus air electrode was fabricated with a void-rich, superaerophobic oxygen evolution reaction catalytic layer and a dense superhydrophobic oxygen reduction reaction catalytic layer. The ZAB using the Janus air electrode exhibits a low voltage gap of 0.78 V for charging and discharging at 10 mA cm^-2, and it can stably work for more than 1 month (1100 cycles) with the decay of only about 10%. Wettability analyses revealed that the Janus superwetting structure provides good electrolyte contact, improves the mass transfer of O₂, and prevents electrolyte leakage and flooding, leading to the high performance. These results suggest the advantage of the Janus electrode in reversible energy-converting devices.

Synergistic Effect of Co9S8 and FeS2 Inlaid on N-Doped Carbon Nanofibers toward a Bifunctional Catalyst for Zn-Air Batteries

The development of economical and energy-efficient electrocatalysts is essential for the wide-scale application of secondary zinc-air batteries (ZABs). Herein, we prepared Co₉S₈ and FeS₂ nanoparticles inlaid on N-doped carbon nanofibers (Co₉S₈-FeS₂@N-CNFs), which were derived from the in situ growth of Fe-doped ZIF-67 nanosheet arrays on electrospun nanofibers and a subsequent sulfidation process. The Co₉S₈-FeS₂@N-CNFs display excellent electrocatalytic performances for OER (E_j=10, 330 mV) and ORR (E_1/2, 0.80 V) as well as a smaller charge and discharge gap (ΔE, 0.76 V) in KOH electrolyte, allowing it to be employed as an attractive air cathode bifunctional catalyst for secondary ZABs. The electrocatalytic performance of the composite materials (Co₉S₈-FeS₂@N-CNFs) is obviously better than that of the single-component materials (FeS₂@N-CNFs and Co₉S₈@N-CNFs). The improved catalytic performance is mainly attributed to the synergistic effect of the two transition-metal sulfides and the optimization of the structure. Furthermore, the peak power density of the assembled aqueous/solid-state ZABs based on Co₉S₈-FeS₂@N-CNFs can reach 214 and 91 mW cm^-2 with excellent stability, respectively, which outperforms the ones based on commercial precious-metal-based catalysts. We anticipate that our work will provide new inspiration for the design of MOF-derived sulfides as multifunctional catalysts.