Recent progress of self-supported air electrodes for flexible Zn-air batteries

Smart wearable devices are regarded to be the next prevailing technology product after smartphones and smart homes,and thus there has recently been rapid development in flexible electronic energy storage devices.Among them,flexible solid-state zinc-air batteries have received widespread attention because of their high energy density,good safety,and stability.Efficient bifunctional oxygen electrocatalysts are the primary consideration in the development of flexible solid-state zinc-air batteries,and self-supported air cathodes are strong candidates because of their advantages including simplified fabrication process,reduced interfacial resistance,accelerated electron transfer,and good flexibility.This review outlines the research progress in the design and construction of nanoarray bifunctional oxygen electrocatalysts.Starting from the configuration and basic principles of zinc-air batteries and the strategies for the design of bifunctional oxygen electrocatalysts,a detailed discussion of self-supported air cathodes on carbon and metal substrates and their uses in flexible zinc-air batteries will follow.Finally,the challenges and opportunities in the development of flexible zinc-air batteries will be discussed.

A review on system and materials for aqueous flexible metal-air batteries

The exploration of aqueous flexible metal-air batteries with high energy density and durability has attracted many research efforts with the demand for portable and wearable electronic devices.Aqueous flexible metal-air batteries feature Earth-abundant materials,environmental friendliness,and operational safety.Each part of one metal-air battery can significantly affect the overall performance.This review starts with the fundamental working principles and the basic battery configurations and then highlights on the common issues and the recent advances in designing high-performance metal electrodes,solid-state electrolytes,and air electrodes.Bifunctional oxygen electrocatalysts with high activity and long-term stability for constructing efficient air electrodes in flexible metal-air batteries are summarized including metal-free carbon-based materials and nonprecious Co/Fe-based materials(alloys,metal oxides,metal sulfites,metal phosphates,metal nitrates,single-site metal-nitrogen-carbon materials,and composites).Finally,a perspective is provided on the existing challenges and possible future research directions in optimizing the performance and lifetime of the flexible aqueous solid-state metal-air batteries.

Bifunctional air electrodes for flexible rechargeable Zn-air batteries

Flexible rechargeable Zn-air batteries are considered as one of the most promising battery systems to drive flexible and wearable electronic devices owing to their high safety,high gravimetric energy density,low self-discharge and low cost.One of the key challenges is to develop air electrodes with high performance and high mechanical flexibility.This minireview discusses the recent progress in the design and fabrication of flexible air electrodes.It focuses on the latest innovations in bifunctional oxygen reduction reaction and oxygen evolution reaction electrocatalysts,mainly including carbon-based materials(e.g.,heteroatom-doped carbon,metal-nitrogen moieties doped carbon),metal oxides(e.g.,spinel oxides,perovskite oxides)and their composites.It aims to provide an insight into the structureproperty relationship of bifunctional catalysts.We also discuss the challenges and future perspectives.

Recent Advances on MOF Derivatives for Non-Noble Metal Oxygen Electrocatalysts in Zinc-Air Batteries

Oxygen electrocatalysts are of great importance for the air electrode in zinc-air batteries(ZABs).Owing to the high specific surface area,controllable pore size and unsaturated metal active sites,metal-organic frameworks(MOFs)derivatives have been widely studied as oxygen electrocatalysts in ZABs.To date,many strategies have been developed to generate efficient oxygen electrocatalysts from MOFs for improving the performance of ZABs.In this review,the latest progress of the MOF-derived non-noble metal-oxygen electrocatalysts in ZABs is reviewed.The performance of these MOF-derived catalysts toward oxygen reduction,and oxygen evolution reactions is discussed based on the categories of metal-free carbon materials,single-atom catalysts,metal cluster/carbon composites and metal compound/carbon composites.Moreover,we provide a comprehensive overview on the design strategies of various MOF-derived non-noble metal-oxygen electrocatalysts and their structure-performance relationship.Finally,the challenges and perspectives are provided for further advancing the MOF-derived oxygen electrocatalysts in ZABs.

High-Entropy Perovskite Oxide: A New Opportunity for Developing Highly Active and Durable Air Electrode for Reversible Protonic Ceramic Electrochemical Cells

Reversible proton ceramic electrochemical cell(R-PCEC)is regarded as the most promising energy conversion device,which can realize efficient mutual conversion of electrical and chemical energy and to solve the problem of large-scale energy storage.However,the development of robust electrodes with high catalytic activity is the main bottleneck for the commercialization of R-PCECs.Here,a novel type of high-entropy perovskite oxide consisting of six equimolar metals in the A-site,Pr_(1/6)La_(1/6)Nd_(1/6)Ba_(1/6)Sr_(1/6)Ca_(1/6)CoO_(3−δ)(PLN-BSCC),is reported as a high-performance bifunctional air electrode for R-PCEC.By harnessing the unique functionalities of multiple ele-ments,high-entropy perovskite oxide can be anticipated to accelerate reaction rates in both fuel cell and electrolysis modes.Especially,an R-PCEC utilizing the PLNBSCC air electrode achieves exceptional electrochemical performances,demonstrating a peak power density of 1.21 W cm^(−2)for the fuel cell,while simultaneously obtaining an astonishing current density of−1.95 A cm^(−2)at an electrolysis voltage of 1.3 V and a temperature of 600℃.The significantly enhanced electrochemical performance and durability of the PLNBSCC air electrode is attributed mainly to the high electrons/ions conductivity,fast hydration reactivity and high configurational entropy.This research explores to a new avenue to develop optimally active and stable air electrodes for R-PCECs.

Air Electrode Supported Manganese Electrocatalysts for Neutral Electrolyte Magnesium Air Cell

X The catalysts of air electrode were prepared by sintering the active carbon loaded with manganese nitrate and potassium permanganate at 360 ℃ . The air electrode was made up of a catalyst layer, a waterproof and gas-permeable layer, a current collecting substrate and a second wa-terproof and gas-permeable layer. The cell was assembled by the air electrode, pure magnesium anode and 10% NaCl solution used as electrolyte. The microstructures of air electrodes before and after discharging were characterized by SEM. The electrochemical behaviors of the air electrodes were determined by means of polarization curves, volt-ampere curves and constant current discharge curves. The polarization voltage of air electrode is-173 mV （vs SCE） at the current density of 50 mA/cm2. The air electrodes exhibits good activity and stability in neutral electrolyte. The magnesium-air cell could work at 5 W for more than 7 h.

Studies of Several Systems with Air Electrode Electro-Synthesizing H_2O_2 on the Spot for Degr ading Aniline in Aqueous

A gas diffusion electrode (air electrode) with a high current efficiency of electro\|synthesizing H 2 O 2 using O 2 in air was prepared. The several systems with air electrode as cathode of ele ctro\|synthesizing H 2 O 2 on the reaction spot for degrading aniline in aqueous--electro\|Fenton system, photo\|excitation electro\|H 2 O 2 system and photo\|electro\|Fenton system, were developed. Th e rates of decomposition of H 2 O 2 and mineralization of anil ine were experimentally measured respectively under different conditions, and th e results indicated there has an excellent parallel relation between decompositi on rate of H 2 O 2 and mineralization rate of aniline. Especia lly, photo\|electro\|Fenton system, where H 2 O 2 is decompose d the fastest, is the best system of oxidizing and degrading organic toxicants. Compared photo\|electro\|Fenton system with photo\|Fenton system, important role is revealed in the interface of air electrode. In this paper, the mineralizatio n mechanism of aniline in the photo\|electro\|Fenton system was also discussed.

A Method of Using a Carbon Fiber Spiral-Contact Electrode to Achieve Atmospheric Pressure Glow Discharge in Air

During discharge, appropriately changing the development paths of electron avalanches and increasing the number of initial electrons can effectively inhibit the formation of filamentary discharge. Based on the aforementioned phenomenon, we propose a method of using microdischarge electrodes to produce a macroscopic discharge phenomenon. In the form of an asymmetric structure composed of a carbon fiber electrode, an electrode structure of carbon fiber spiral-contact type is designed to achieve an atmospheric pressure glow discharge in air, which is characterized by low discharge voltage, low energy consumption, good diffusion and less ozone generation.

Research progress of transition metal compounds as bifunctional catalysts for zinc-air batteries

Zinc-air batteries(ZABs)are widely studied because of their high theoretical energy density,high battery voltage,environmental protection,and low price.However,the slow kinetics of oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)on the air electrode limits the further application of ZABs,so that how to develop a cheap,efficient,and stable catalyst with bifunctional catalytic activity is the key to solving the development of ZABs.Transition metal compounds are widely used as cathode materials for ZABs due to their low cost,high electrocatalytic activity,and stable structure.This review summarizes the research progress of transition metal compounds as bifunctional catalysts for ZABs.The development history,operation principle,and mechanism of ORR and OER reactions are introduced first.The application and development of transition metal compounds as bifunctional catalysts for ZABs in recent years are systematically introduced,including transition metal oxides(TMOs),transition metal nitrides(TMNs),transition metal sulfides(TMSs),transition metal carbides(TMCs),transition metal phosphates(TMPs),and others.In addition,the shortcomings of transition metal compounds as bifunctional catalysts for ZABs were summarized and reasonable design strategies and improvement measures were put forward,aiming at providing a reference for the design and construction of high-performance ZABs cathode materials.Finally,the challenges and future in this field are discussed and prospected.

The application of carbon materials in nonaqueous Na‐O2 batteries

Na‐O2 batteries are advantageous as the candidates of next‐generation electric vehicles due to their ultrahigh theoretical energy density and have attracted enormous attention recently.Tremendous efforts have been devoted to improve the Na‐O2 battery performance by designing advanced electrodes with various carbonbased materials.Carbon materials used in Na‐O2 batteries not only function as the air electrode to provide active sites and accommodate discharge products but also as Na anode protectors against dendrite growth and chemical/electrochemical corrosion.In this review,we mainly focus on the application of various carbonbased materials in Na‐O2 batteries and highlight their advances.The scientific understanding on the fundamental design of the material microstructure and chemistry in relation to the battery performance are summarized.Finally,perspectives on enhancing the overall battery performance based on the optimization and rational design of carbon‐based cell components are also briefly anticipated.

An Effective Novel ReactionSystem For The Photo Degradation of Aqueous Organic Pollutants

A novel reaction system consisted of a supported TiO2 film electrode, a Ru?Ti oxide film electrode and air (oxygen) electrode is reported. The air (oxygen) electrode can provide H2O2 continuously for homogeneous photochemical oxidation reaction on the spot. In this reactor, degradation reaction of aniline occur from interface of TiO2 film to all solution which is irradiated by ultraviolet ray. The degradation rate of aniline was characterized by measuring the change of chemical oxygen demand (COD) in solution under different conditions. It was found that the degradation rate of aniline in the novel system increased apparently as compared with single heterogeneous photocatalysis and homogeneous photochemistry system. It can be explained in terms of combining acts of heterogeneous photocatalysis and homogeneous photochemistry.

High-entropy perovskite oxide BaCo_(0.2)Fe_(0.2)Zr_(0.2)Sn_(0.2)Pr_(0.2)O_(3-δ) with triple conduction for the air electrode of reversible protonic ceramic cells

Reversible protonic ceramic cells(RPCCs) show great potential as new-generation energy conversion and storage devices. However, the mature development of RPCCs is seriously hindered by the inactivity and poor stability of air electrodes exposed to concentrated vapor under operating conditions. Herein, we report a high-entropy air electrode with the composition BaCo_(0.2)Fe_(0.2)Zr_(0.2)Sn_(0.2)Pr_(0.2)O_(3-δ)(BCFZSP), which shows integrated electronic, protonic and oxygenic conduction in a single perovskite phase and excellent structural stability in concentrated steam. Such triple conduction can spread the electrochemically active sites of the air electrode to the overall electrode surface, thus optimizing the kinetics of the oxygen reduction and evolution reactions(0.448 Ω cm^(2) of polarization resistance at 550℃). As-prepared RPCCs with a BCFZSP air electrode at 600℃ achieved a peak power density of 0.68 W/cm^(2) in fuel-cell mode and a current density of 0.92 A/cm^(2) under a 1.3 V applied voltage in electrolysis mode. More importantly, the RPCCs demonstrate an encouragingly high stability during 120 h of reversible switching between the fuelcell and electrolysis modes. Given their excellent performance, high-entropy perovskites can be promising electrode materials for RPCCs.

Constructing a stable cobalt-nitrogen-carbon air cathode from coordinatively unsaturated zeolitic-imidazole frameworks for rechargeable zinc-air batteries

Zeolitic-imidazole frameworks(ZIFs)derivations have widely emerged as an efficient air cathode of zinc-air batteries(ZABs)due to excellent bifunctional oxygen electrocatalysis performance.However,they are not stable enough for long-term operation of rechargeable ZABs because of weak association with current collector,especially under bending conditions for flexible ZAB devices.Here,we show that by purposely designing coordinatively unsaturated ZIFs via a facile morphology regulation,which can be chemically linked on acid-treated carbon cloth,a stable Co-N-C air cathode is therefore derived where Co nanoparticles(NPs)are uniformly confined within the Co-N-C matrix on carbon cloth(Co/Co-N-C/CC).Specifically,when without being stabilized from carbon cloth,the pyrolysis of ZIFs with different unsaturated coordination levels has a negligible impact on the bifunctional oxygen-catalyzed performance.The optimal Co/Co-N-C/CC catalyst assembled ZAB possesses a large open circuit voltage of 1.415 V and a high peak power density of 163 mW·cm^(−2) as well as excellent cycling durability upon 630 discharge–charge cycles with 61%voltage efficiency remained,largely exceeding those of a benchmark Pt/C-IrO_(2) catalyst assembled ZAB.The synergy between Co NPs and active Co-N-C sites via electronic interaction induces the outstanding bifunctional oxygen-catalyzed activity and cathode performance.The present work highlights the importance of unsaturated coordination structures in ZIFs precursors for the performance of derived nanostructures in integrated electrodes.

Wood-derived integrated air electrode with Co-N sites for rechargeable zinc-air batteries

The sluggish reaction kinetics in oxygen reduction reaction(ORR)is one of the bottlenecks in next generation energy conversion systems.The integrated design strategy based on simultaneously constructing active sites and forming porous carbon network will address this concern by facilitating charge exchange,mass transfer and electron transportation.In this article,a three-dimensional integrated air electrode(Co-N@ACS)containing Co-N sites and hierarchically porous carbon is fabricated via growth of Co-doped ZIF-8 in activated wood substrate and synchronous pyrolysis.The optimized integrated air electrodes exhibit ultrahigh ORR activity(E_(1/2)=0.86 V).Co-N sites provide outstanding ORR activity,and hierarchically porous structures facilitate oxygen diffusion and electrolyte penetration.Aqueous zinc-air battery assembled with Co-N@ACS possesses open-circuit voltage of 1.46 V,peak power density of 155 mW cm^(-2) and long-term stability of 540 cycles(180 h).Solid-state zinc-air battery assembled with Co-N@ACS shows open-circuit voltage up to 1.36 V and low charge-discharge voltage gap(0.8 V).This design strategy paves the way for the conversion of wood biomass to integrated air electrodes and catalytically active carbon for next generation energy storage and conversion devices.

A Novel Composite Electrode of Photocatalyst-TiO_2/C Loading on the Surface of the Air (Oxygen) Electrode

The methods for preparing the H_2O_2 generating air (oxygen) electrode andthe composite electrode of photocatalyst-TiO_2/C loading on the surface of the air (oxygen)electrode were introduced. In the case of the composite electrode, the current efficiency ofelectro-generated H_2O_2 is higher than 80% (J ≤ 15 mA/cm^2). The degradation of aniline was usedas an example to measure the influence of the composite electrode and compared with the system inwhich the air (oxygen) electrode and the photocatalyst-TiO_2 were separated. The results confirmedthat the composite electrode played an active role on accelerating the degradation rate of aniline.According to the measurement of the polarization curves of composite electrode and TiO_2 photoanode, and of the adsorbing amount of aniline on the surface of the composite electrode, theprinciple of descending the recombination rate of photo-generated electron and hole and of enhancingthe oxidation rate of organic molecule was described. The mechanism about the degradation ofaniline was also discussed.