Construction of mass-transfer channel in air electrode with bifunctional catalyst for rechargeable zinc-air battery

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.

Nanoemulsion-Coated Ni-Fe Hydroxide Self-Supported Electrode as an Air-Breathing Cathode for High-Performance Zinc-Air Batteries

To improve the energy conversion efficiency and durability of zinc-air batteries (ZABs) for large-scale implementations, here we propose an "air-breathing" strategy to significantly enlarge triple-interfaces with intensified mass transfer. By dip-coating the aerophilic perfluorochemical compounds (PFC) and amphiphilic ionomers into the self-supported electrodes, (1) the high solubility of O₂ in the PFC nanoemulsions greatly increases triple-phase boundaries and facilitates the efficient supply/removal of O₂ from the electrolyte; (2) the ionomers with hydrophobic and hydrophilic functionalities enable fast gas, water, and ion transport to the triple-phase boundaries; and (3) the self-supported electrode without binder ensures fast electron transfer while the firm integration prevents catalyst shedding. By applying this strategy, the ZABs show a high power density of 115 mW cm^-2 and a narrow discharge/charge gap of 0.64 V at 10 mA cm^-2 and a long-cycling durability (over 1000 h). This work provides a universal approach to promote gas-evolving reactions for electrochemical applications.

Frontiers and Structural Engineering for Building Flexible Zinc-Air Batteries

With the development of flexible devices, the demand for wearable power sources has increased and gradually become imperative. Zinc-air batteries (ZABs) have attracted lots of research interest due to their high theoretical energy density and excellent safety properties, which can meet the wearable energy supply requirements. Here, the flexibility of energy storage devices is discussed first, followed by the chemistries and development of flexible ZABs. The design of flexible electrodes, the properties of solid-state electrolytes (SSEs), and the construction of deformable structures are discussed in depth. The researchers working on flexible energy storage devices will benefit from the work.

Toward Flexible Zinc-Air Batteries with Self-Supported Air Electrodes

The compelling demand for higher energy performance, flexibility, and miniaturization is the main driving force of the energy storage and conversion industry's quest for flexible devices based on new integration and fabrication process. Herein, the recent advances on the development of flexible zinc-air batteries based on self-supported air electrodes are summarized, focusing on the multiscale and systematic design principles for the design of flexible air electrodes. With the electrocatalytic activity regulation and structural engineering strategies, the rational design of self-supported air electrodes is discussed in integrated devices to underpin the good flexibility for wearable requirement. The perspectives on promising developments of flexible zinc-air batteries and the accumulated knowledge from other flexible devices are also addressed for promoting the advances on flexible zinc-air batteries.

The Zinc-Air Battery Performance with Ni-Doped MnO2 Electrodes

A rechargeable zinc-air battery shows great promise because of its high energy density, low cost, greater safety, and its environment-friendly properties. However, rechargeable zinc-air battery development has been hindered by the lack of a satisfactory bi-functional electrode. In this research, we report on a solution which uses electro-deposition to dope nickel into manganese on the stainless-steel mesh. The result shows the hydroxyl group on the prepared samples improving its oxygen reduction reaction and oxygen evolution reaction performance, as well as boosting the ion diffusion rate and stabilizing the zinc-air battery charge-discharge performance (overall potential gap dropped from 0.84 V to 0.82 V after 1000 cycles). This study contributes to our understanding of a new method for the improvement of bi-functional electrodes.

Progress of carbon-based electrocatalysts for flexible zinc-air batteries in the past 5 years: recent strategies for design, synthesis and performance optimization

The increasing popularity of wearable electronic devices has led to the rapid development of flexible energy conversion systems. Flexible rechargeable zinc-air batteries (ZABs) with high theoretical energy densities demonstrate significant potential as next-generation flexible energy devices that can be applied in wearable electronic products. The design of highly efficient and air-stable cathodes that can electrochemically catalyze both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are highly desirable but challenging. Flexible carbon-based catalysts for ORR/OER catalysis can be broadly categorized into two types: (i) self-supporting catalysts based on the in situ modification of flexible substrates; (ii) non-self-supporting catalysts based on surface coatings of flexible substrates. Methods used to optimize the catalytic performance include doping with atoms and regulation of the electronic structure and coordination environment. This review summarizes the most recently proposed strategies for the synthesis of designer carbon-based electrocatalysts and the optimization of their electrocatalytic performances in air electrodes. And we significantly focus on the analysis of the inherent active sites and their electrocatalytic mechanisms when applied as flexible ZABs catalysts. The findings of this review can assist in the design of more valuable carbon-based air electrodes and their corresponding flexible ZABs for application in wearable electronic devices.

NiMOF-derived oxygen vacancy rich NiO with excellent capacitance and ORR/OER activities as a cathode material for Zn-based hybrid batteries

An Ni-Zn battery is a distinguished member in the family of closed Zn-based batteries due to its ideal power density and voltage. However, when it is employed as a power supply for electric vehicles, its defects in terms of specific capacitance and energy density become obvious. Herein, to resolve this problem, a hybrid battery system was created through a combination of Ni-Zn and Zn-air batteries at the cell level. In a hybrid battery system, oxygen vacancy rich NiO with S,N co-modified mesoporous carbon as a matrix was used as the cathode material. This cathode material showed a high specific capacitance of 202.1 mA h g-1 at 1.0 A g-1. When the current density reduces to 20 A g-1, this value decreases to 130.2 mA h g-1, which implies that 64.4% of specific capacitance was retained. It also exhibits excellent OER and ORR activities. For the hybrid battery system, when the discharge process was carried out at 1 mA cm-2, there were two voltage plateaus at 1.72 and 1.12 V, which originated from Ni-Zn and Zn-air, respectively. In this case, its specific capacitance and energy density reaches 800.3 mA h g-1 and 961 W h kg-1, respectively. The hybrid battery also possesses perfect stability during multi-cycle charge-discharge tests. The construction of this hybrid battery system develops a new road to prepare a power supply device with high performance.

Recent Advances on Self-Supported Arrayed Bifunctional Oxygen Electrocatalysts for Flexible Solid-State Zn-Air Batteries

Flexible solid-state Zn-air batteries have been rapidly developed benefiting from the uprising demand for wearable electronic devices, wherein the air electrode integrated with efficient bifunctional oxygen electrocatalysts plays an important role to achieve high performance. Binder-free self-supported bifunctional catalysts can provide large active surface area, fast electron transport path, easy ion diffusion, and excellent structural stability and flexibility, thus acting as promising flexible air cathodes. In this review, recent advances on the application of nanoarrayed electrocatalysts as air cathodes in flexible Zn-air batteries are reviewed. Especially, various types of bifunctional oxygen electrocatalysts, including carbonaceous material arrays, transition metal compound arrays, transition metal/carbon arrays, transition metal compound/carbon arrays, and other hybrid arrays, are discussed. The applications of flexible Zn-air batteries with two configurations (i.e., planar stacks and cable fibers) are also introduced. Finally, perspectives on the optimization of arrayed air cathodes for future development to achieve high-performance flexible Zn-air batteries are shared.

A Rechargeable Zn-Air Battery with High Energy Efficiency and Long Life Enabled by a Highly Water-Retentive Gel Electrolyte with Reaction Modifier

Tremendous effort have recently been made in optimizing the air catalysts of flexible zinc-air batteries (ZABs). Unfortunately, the bottleneck factors in electrolytes that largely limit the working life and energy efficiency of ZABs have long been relatively neglected. Herein, an alkaline gel polymer electrolyte (GPE) is fabricated through multiple crosslinking reactions among poly(vinyl alcohol) (PVA), poly(acrylic acid), and graphene oxide followed by intense uptake of an alkali and the KI reaction modifier. The prepared GPE exhibits essentially improved properties compared to traditional PVA gel electrolyte in terms of mechanical strength, ionic conductivity, and water retention capability. In addition, the introduced reaction modifier I^- in the GPE changes the path of the conventional oxygen evolution reaction, leading to a more thermodynamically favorable path. The optimized GPE enables flexible ZABs exhibiting an exceptionally low charge potential of 1.69 V, a long cycling time of 200 h, a high energy efficiency of 73%, and rugged reliability under different extreme working conditions. Moreover, the successful integration of ZABs in a variety of real wearable electronic devices demonstrates their excellent practicability as flexible power sources.

A Dendrite-Resistant Zinc-Air Battery

Zinc-air batteries (ZABs) have drawn widespread attention for their high energy densities, abundant raw materials, and low cost. However, the issues of metal dendrite formation and air electrode failure have been impeding the development and application of ZABs. Herein, we designed a novel dendrite-resistant ZAB system by adopting multiphase electrolytes to conduct the zinc deposition and the oxygen evolution reaction. The oxygen reduction reaction electrode is kept out of the zinc deposition region to extend the lifespan. The ZABs show an energy density of 1,050.9 Wh kg⁻¹ based on the mass of zinc consumption, with an average Coulombic efficiency of ∼97.4% in 2,000 h discharge and charge cycling. More impressively, even if a short circuit occurs while charging, the battery can maintain the cycle performance without irreversible failure, which is conducive to the reliability of battery modules and its application in other energy storage/conversion devices.

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The ZAB displays an ultralong cycle life (10,000 cycles) at high current density

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The ZAB shows high energy densities and an average Coulombic efficiency of ∼97.4%

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Our ZABs compose energy storage module showing the peak power density of 280.8 mW cm⁻²

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The ZAB has excellent cycle performance, and it can be recovered in time of short circuit

Rational Design of Spinel Oxide Nanocomposites with Tailored Electrochemical Oxygen Evolution and Reduction Reactions for ZincAir Batteries

The unique physical and chemical properties of spinels have made them highly suitable electrocatalysts in oxygen evolution reaction and oxygen reduction reaction (OER & ORR). Zinc–air batteries (ZABs), which are safer and more cost-effective power sources than commercial lithium-ion batteries, hinge on ORR and OER. The slow kinetics of the air electrode reduce its high theoretical energy density and specific capacity, which limits its practical applications. Thus, tuning the performance of the electrocatalyst and cathode architecture is vital for improving the performance of ZABs, which calls for exploring spinel, a material that delivers improved performance. However, the structure–activity relationship of spinel is still unclear because there is a lack of extensive information about it. This study was performed to address the promising potential of spinel as the bifunctional electrocatalyst in ZABs based on an in-depth understanding of spinel structure and active sites at the atomic level.

Construction of a Janus MnO2-NiFe Electrode via Selective Electrodeposition Strategy as a High-Performance Bifunctional Electrocatalyst for Rechargeable Zinc-Air Batteries

MnO₂ has been considered as the most promising bifunctional electrocatalyst toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Despite their highly active ORR performance, the OER catalytic activity of MnO₂ species is still far from satisfying. Herein, for the first time, highly active OER catalytic NiFe layered double hydroxides (NiFe LDHs) are combined with MnO₂ via a selective electrodeposition method to form a Janus electrode in which the MnO₂ and NiFe LDHs are in situ grown on two sides of a porous nickel foam (MnO₂-NiFe/Ni). The MnO₂-NiFe/Ni electrode exhibits excellent bifunctional catalytic activity and stability for both ORR and OER compared to bare MnO₂ on account of the rational design of the Janus bifunctional configuration separating OER and ORR active materials. Moreover, such a Janus MnO₂-NiFe air electrode endows the zinc-air battery with better cycling stability and energy efficiency than the bare MnO₂ electrode. Our work demonstrates a novel Janus electrode configuration to design high-performance electrocatalysts for energy storage and conversion applications.