Development of flexible zinc–air battery with nanocomposite electrodes and a novel separator

In this paper, we present the development of flexible zinc–air battery. Multiwalled carbon nanotubes(MWCNTs) were added into electrodes to improve their performance. It was found that MWCNTs were effective conductive additive in anode as they bridged the zinc particles. Poly(3,4-ethylenedioxythiophene)polystyrene sulfonate(PEDOT:PSS) was applied as a co-binder to enhance both the conductivity and flexibility. A poly(acrylic acid)(PAA) and polyvinyl alcohol(PVA) coated paper separator was used to enhance the battery performance where the PVP–PAA layer facilitated electrolyte storage. The batteries remained functional under bending conditions and after bending. Multiple design optimizations were also carried out for storage and performance purposes.

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.

Self-supported VO_(2) on polydopamine-derived pyroprotein-based fibers for ultrastable and flexible aqueous zinc-ion batteries

A conventional electrode composite for rechargeable zinc-ion batteries(ZIBs)includes a binder for strong adhesion between the electrode material and the current collector.However,the introduction of a binder leads to electrochemical inactivity and low electrical conductivity,resulting in the decay of the capacity and a low rate capability.We present a binder-and conducting agent-free VO_(2) composite electrode using in situ polymerization of dopamine on a flexible current collector of pyroprotein-based fibers.The as-fabricated composite electrode was used as a substrate for the direct growth of VO_(2) as a self-supported form on polydopamine-derived pyroprotein-based fibers(pp-fibers@VO_(2)(B)).It has a high conductivity and flexible nature as a current collector and moderate binding without conventional binders and conducting agents for the VO_(2)(B) cathode.In addition,their electrochemical mechanism was elucidated.Their energy storage is induced by Zn^(2+)/H^(+) coinsertion during discharging,which can be confirmed by the lattice expansion,the formation of by-products including Zn_(x)(OTf)_(y)(OH)_(2x−y)·nH_(2)O,and the reduction of V^(4+)to V^(3+).Furthermore,the assembled Zn//pp-fibers@VO_(2)(B) pouch cells have excellent flexibility and stable electrochemical performance under various bending states,showing application possibilities for portable and wearable power sources.

Dynamic stretching–electroplating metal-coated textile for a flexible and stretchable zinc–air battery

A metal electrode is a significant component of a zinc–air battery(ZAB),but the metal material is usually not elastic,which severely restricts the application of flexible and stretchable ZABs in the field of wearable electronic devices.Herein,we report a flexible and stretchable metal-coated textile prepared by a dynamic stretching–electroplating based on a wavy spandex textile substrate.Benefiting from the unique woven and wavy structure,the metal-coated textile shows a high stretchability of 100%and stable conductivity.In situ scanning electron microscope observation during stretching showed that the tensile strain of the metal-coated textile is mainly attributed to the deformation of the microfiber network at the bottom position of the wave structure.In addition,a sodium carboxymethyl cellulose–polyacrylic acid–potassium hydroxide composite hydrogel has been used as the electrolyte.This hydrogel shows excellent ionic conductivity,mechanical properties,and water retention properties,which makes it suitable for the semi-open system of ZAB.Furthermore,a flexible and stretchable sandwich-structure ZAB,assembled using the above-mentioned electrodes and electrolyte,operates stably even under rapid stretching/releasing cycle deformation.Because of its facile preparation and low cost,this flexible and stretchable ZAB is suitable for fabrication of large-area batteries to obtain higher output current and power to drive wearable electronic devices.

Co/CoO heterojunction rich in oxygen vacancies introduced by O_(2) plasma embedded in mesoporous walls of carbon nanoboxes covered with carbon nanotubes for rechargeable zinc-air battery

Herein,Co/CoO heterojunction nanoparticles(NPs)rich in oxygen vacancies embedded in mesoporous walls of nitrogen-doped hollow carbon nanoboxes coupled with nitrogen-doped carbon nanotubes(P-Co/CoOV@NHCNB@NCNT)are well designed through zeolite-imidazole framework(ZIF-67)carbonization,chemical vapor deposition,and O_(2) plasma treatment.As a result,the threedimensional NHCNBs coupled with NCNTs and unique heterojunction with rich oxygen vacancies reduce the charge transport resistance and accelerate the catalytic reaction rate of the P-Co/CoOV@NHCNB@NCNT,and they display exceedingly good electrocatalytic performance for oxygen reduction reaction(ORR,halfwave potential[EORR,1/2=0.855 V vs.reversible hydrogen electrode])and oxygen evolution reaction(OER,overpotential(η_(OER,10)=377mV@10mA cm^(−2)),which exceeds that of the commercial Pt/C+RuO_(2) and most of the formerly reported electrocatalysts.Impressively,both the aqueous and flexible foldable all-solid-state rechargeable zinc-air batteries(ZABs)assembled with the P-Co/CoOV@NHCNB@NCNT catalyst reveal a large maximum power density and outstanding long-term cycling stability.First-principles density functional theory calculations show that the formation of heterojunctions and oxygen vacancies enhances conductivity,reduces reaction energy barriers,and accelerates reaction kinetics rates.This work opens up a new avenue for the facile construction of highly active,structurally stable,and cost-effective bifunctional catalysts for ZABs.

Flexible, Porous, and Metal–Heteroatom?Doped Carbon Nanofibers as Efficient ORR Electrocatalysts for Zn–Air Battery

Developing an e cient and durable oxygen reduction electrocatalyst is critical for clean-energy technology, such as fuel cells and metal–air batteries. In this study, we developed a facile strategy for the preparation of flexible, porous, and well-dispersed metal–heteroatom-doped carbon nanofibers by direct carbonization of electrospun Zn/Co-ZIFs/PAN nanofibers(Zn/Co-ZIFs/PAN). The obtained Zn/Co and N co-doped porous carbon nanofibers carbonized at 800 °C(Zn/Co–N@PCNFs-800) presented a good flexibility, a continuous porous structure, and a superior oxygen reduction reaction(ORR) catalytic activity to that of commercial 20 wt% Pt/C, in terms of its onset potential(0.98 V vs. RHE), half-wave potential(0.89 V vs. RHE), and limiting current density(-5.26 mA cm^(-2)). In addition, we tested the suitability and durability of Zn/Co–N@PCNFs-800 as the oxygen cathode for a rechargeable Zn–air battery. The prepared Zn–air batteries exhibited a higher power density(83.5 mW cm^(-2)), a higher specific capacity(640.3 mAh g^(-1)), an excellent reversibility, and a better cycling life than the commercial 20 wt% Pt/C + RuO_2 catalysts. This design strategy of flexible porous non-precious metal-doped ORR electrocatalysts obtained from electrospun ZIFs/polymer nanofibers could be extended to fabricate other novel, stable, and easy-to-use multi-functional electrocatalysts for clean-energy technology.

Bifunctional Oxygen Electrocatalyst of Mesoporous Ni/NiO Nanosheets for Flexible Rechargeable Zn–Air Batteries

One approach to accelerate the stagnant kinetics of both the oxygen reduction and evolution reactions(ORR/OER)is to develop a rationally designed multiphase nanocomposite,where the functions arising from each of the constituent phases,their interfaces,and the overall structure are properly controlled.Herein,we successfully synthesized an oxygen electrocatalyst consisting of Ni nanoparticles purposely interpenetrated into mesoporous NiO nanosheets(porous Ni/NiO).Benefiting from the contributions of the Ni and NiO phases,the well-established pore channels for charge transport at the interface between the phases,and the enhanced conductivity due to oxygen-deficiency at the pore edges,the porous Ni/NiO nanosheets show a potential of 1.49 V(10 mA cm^-2)for the OER and a half-wave potential of 0.76 V for the ORR,outperforming their noble metal counterparts.More significantly,a Zn-air battery employing the porous Ni/NiO nanosheets exhibits an initial charging-discharging voltage gap of 0.83 V(2 mA cm^-2),specific capacity of 853 mAh gZn^-1 at 20 mA cm^-2,and long-time cycling stability(120 h).In addition,the porous Ni/NiO-based solid-like Zn-air battery shows excellent electrochemical performance and flexibility,illustrating its great potential as a next-generation rechargeable power source for flexible electronics.

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.

Recently advances in flexible zinc ion batteries

Flexible batteries are key component of wearable electronic devices.Based on the requirements of medical and primary safety of wearable energy storage devices,rechargeable aqueous zinc ion batteries(ZIBs)are promising portable candidates in virtue of its intrinsic safety,abundant storage and low cost.However,many inherent challenges have greatly hindered the development in flexible Zn-based energy storage devices,such as rigid current collector and/or metal anode,easily detached cathode materials and a relatively narrow voltage window of flexible electrolyte.Thus,overcoming these challenges and further developing flexible ZIBs are inevitable and imperative.This review summarizes the most advanced progress in designs and discusses of flexible electrode,electrolyte and the practical application of flexible ZIBs in different environments.We also exhibit the heart of the matter that current flexible ZIBs faces.Finally,some prospective approaches are proposed to address these key issues and point out the direction for the future development of flexible ZIBs.

High rate and ultralong life flexible all-solid-state zinc ion battery based on electron density modulated NiCo_(2)O_(4) nanosheets

The development of zinc ion batteries (ZIBs) with large capacity,high rate,and durable cathode material is a crucial and urgent task.Ni Co_(2)O_(4)(NCO) has received ever-growing interest as a potential cathode material for ZIBs,owing to the high theoretical capacity,rich source,cost-effective,and versatile redox nature.However,due to the slow dynamics of the NCO electrodes,its practical application in highperformance systems is severely limited.Herein,we report an electron density modulated NCO nanosheets (N-NCO NSs) with high-kinetics Zn^(2+)-storage capability as an additive-free cathode for flexible all-solid-state (ASS) ZIBs.By virtue of the enhanced electronic conductivity,improved reaction kinetics,and increased active sites,the optimized N-NCO NSs electrode delivers a high capacity of 357.7 m Ah g^(-1)at 1.0 A g^(-1)and a superior rate capacity of 201.4 m Ah g^(-1)at 20 A g^(-1).More importantly,a flexible ASS ZIBs device is manufactured using a solid polymer electrolyte of a poly (vinylidene fluoride hexafluoropropylene)(PVDF-HFP) film.The flexible ASS ZIBs device shows superb durability with 80.2%capacity retention after 20,000 cycles and works well in the range of-20–70℃.Furthermore,the flexible ASS ZIBs achieves an impressive energy density as high as 578.1 W h kg^(-1)with a peak power density of 33.6 k W kg^(-1),substantially outperforming those latest ZIBs.This work could provide valuable insights for constructing high-kinetics and high-capability cathodes with long-term stability for flexible ASS ZIBs.

Flexible Zinc-Manganese Dioxide Alkaline Batteries Based on Kelp Electrolytes

Flexible energy-storage devices play a critical role in the development of portable, flexible and wearable electronics. In addition, biological materials including plants or plant-based materials are known for their safety, biodegradability, biocompatibility, environmental benignancy, and low cost. With respect to these advances, a flexible alkaline zinc-manganese dioxide (Zn-MnO2) battery is fabricated with a kelp-based electrolyte in this study. To the best of our knowledge, pure kelp is utilized as a semi-solid electrolyte for flexible Zn-MnO2 alkaline batteries for the first time, with which the as-assembled battery exhibited a specific capacity of 60 mA·h and could discharge for 120 h. Furthermore, the as-assembled Zn-MnO2 battery can be bent into a ring-shape and power a light-emitting diode screen, showing promising potential for the practical application in the future flexible, portable and biodegradable electronic devices.

Nitrogen-doped carbon nanotube encapsulating cobalt nanoparticles towards efficient oxygen reduction for zinc–air battery

Nitrogen-doped carbon materials encapsulating 3 d transition metals are promising alternatives to replace noble metal Pt catalysts for efficiently catalyzing the oxygen reduction reaction(ORR). Herein, we use cobalt substituted perfluorosulfonic acid/polytetrafluoroethylene copolymer and dicyandiamide as the pyrolysis precursor to synthesize nitrogen-doped carbon nanotube(N–CNT) encapsulating cobalt nanoparticles hybrid material. The carbon layers and specific surface area of N–CNT have a critical role to the ORR performance due to the exposed active sites, determined by the mass ratio of the two precursors. The optimum hybrid material exhibits high ORR activity and stability, as well as excellent performance and durability in zinc–air battery.

FeCo alloy/N,S dual-doped carbon composite as a high-performance bifunctional catalyst in an advanced rechargeable zinc-air battery

The rational design and development of cost-effective,high-performance,and stable bifunctional oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)electrocatalysts are essential for rechargeable zinc-air batteries.Herein,a novel FeCo composite composed of alloy nanoparticles embedded in an N,S dual-doped carbon matrix(FeCo/NSC)was prepared via one-step carbonization of amphiphilic dodecanethiol-metal salts wrapped in carbon nitride(C_(3)N_(4)).The compact combination of dual metalalloys and dual-doped carbon endowed the composite with the active sites for the ORR and OER,achieving efficient electrical transmission and highly efficient bifunctional catalytic performance.The obtained FeCo-1/NSC catalyst exhibited excellent electrocatalytic activity with a half-wave potential of 0.82 V(vs.RHE)for the ORR and a low overpotential of 0.325 V at 10 mA cm^(-2) for the OER.The liquid Zn-air battery with FeCo-1/NSC as an air electrode displayed excellent charge-discharge performance,high power density,and robust charge-discharge stability for 150 h compared to the 20%Pt/C+RuO_(2) counterpart.Furthermore,the FeCo-1/NSC-based flexible solid-state Zn-air battery exhibited a higher power density and good charge-discharge stability over 10 h of operation.Thus,a promising strategy for bifunctional electrocatalyst development as part of rechargeable and wearable Zn-air batteries was provided.

Recent progress of flexible aqueous multivalent ion batteries

Flexible aqueous batteries have been thriving with the growing demand for wearable and portable electrical devices.In particular,flexible aqueous mul tivalent ion batteries(FAMIBs),the charge carriers of which include Zn^(2+),Al^(3+),Mg^(2+),and Ca^(2+),have great potential for development owing to their high safety,high elemental abundance in the Earth's crust,and a multi-electron redox mechanism with a high theoretical specific capacity.Therefore,for a comprehensive understanding of this developing field,it is necessary to summarize the recent research progress of FAMIBs in a timely manner.Herein,the advancements of the state-of-the-art FAMIBs are reviewed,and the prospects toward this field are also proposed.This study focuses on the rational material and configuration design for FAMIBs in recent studies to achieve high battery performances under deformation conditions,which is elaborated on by classification of the anode,cathode,hydrogel electrolyte,and configurations of FAMIBs.Besides,the electrochemical performance of FAMIBs under flexible conditions is also reviewed from the perspective of their working voltage,specific capacity,and cycling stability.Finally,the ap proaches to improve the performance of FAMIBs are comprehensively eval uated,followed by the outlook on the challenges and opportunities in future development of FAMIBs.

Efficient spinel iron‐cobalt oxide/nitrogen‐doped ordered mesoporous carbon catalyst for rechargeable zinc‐air batteries

A robust oxygen‐related electrocatalyst,composed of spinel iron‐cobalt oxide and nitrogen‐dopedordered mesoporous carbon(NOMC),was developed for rechargeable metal‐air batteries.Electrochemicaltests revealed that the optimal catalyst Fe_(0.5)Co/NOMC exhibits superior activity with ahalf‐wave potential of 0.89 V(vs.reversible hydrogen electrode)for the oxygen reduction reactionand an overpotential of 0.31 V at 10 mA cm^(−2)for the oxygen evolution reaction.For demonstration,the catalyst was used in the assembly of a rechargeable zinc‐air battery,which exhibited an exceptionallyhigh energy density of 820 Wh kg−1 at 100 mA cm^(−2),a high power density of 153 mW cm^(−2)at1.0 V,and superior cycling stability up to 432 cycles(144 h)under ambient air.

Co_(2)N Nanoparticles Anchored on N-Doped Active Carbon as Catalyst for Oxygen Reduction Reaction in Zinc–Air Battery

The development of efficient catalytic electrode toward oxygen reduction reaction(ORR)is still a great challenge for the wide use of zinc–air batteries.Herein,Co_(2)N nanoparticles(NPs)anchored on N-doped carbon from cattail were verified with excellent catalytic performances for ORR.The onset and half-wave potentials over the optimal catalyst reach to 0.96 V and 0.84 V,respectively.Current retention rates of 96.8%after 22-h test and 98.8%after running 1600 s were obtained in 1 M methanol solution.Density functional theory simulation proposes an apparently increased electronic states of Co_(2)N in N-doped carbon layer close to the Fermi level.Higher charge density,favorable adsorption,and charge transfer of intermediates originate from the coexistence of Co_(2)N NPs and N atoms in carbon skeleton.The superior catalytic activity of composites also was confirmed in zinc–air batteries.This novel catalytic property and controllable preparation approach of Co_(2)Ncarbon composites provide a promising avenue to fabricate metal-containing catalytically active carbon from biomass.

Test factors affecting the performance of zinc–air battery

Zinc-air batteries provide a great potential for future large-scale energy storage.We assess the test factors that mainly affect the measured power density of the zinc-air battery.By fitting the polarization curves of the zinc-air batteries,we reveal the effect of testing parameters(electrode distance,electrolyte concentration,and oxygen flux)and preparation of catalysts ink on the activation,ohm,and concentration polarizations of the zinc-air battery.Finally,recommendations on evaluating the potentials of non-noblemetal electrocatalysts for applications in zinc-air batteries were given.

3D hollow sphere Co_3O_4/MnO_2-CNTs:Its high-performance bi-functional cathode catalysis and application in rechargeable zinc-air battery

There has been a continuous need for high active, excellently durable and low-cost electrocatalysts for rechargeable zinc-air batteries. Among many low-cost metal based candidates, transition metal oxides with the CNTs composite have gained increasing attention. In this paper, the 3-D hollow sphere MnO_2 nanotube-supported Co_3O_4 nanoparticles and its carbon nanotubes hybrid material(Co_3 O_4/MnO_2-CNTs) have been synthesized via a simple co-precipitation method combined with post-heat treatment. The morphology and composition of the catalysts are thoroughly analyzed through SEM, TEM, TEM-mapping, XRD, EDX and XPS. In comparison with the commercial 20% Pt/C, Co_3O_4/MnO_2,bare MnO_2 nanotubes and CNTs, the hybrid Co_3O_4/MnO_2-CNTs-350 exhibits perfect bi-functional catalytic activity toward oxygen reduction reaction and oxygen evolution reaction under alkaline condition(0.1 M KOH). Therefore, high cell performances are achieved which result in an appropriate open circuit voltage(～1.47 V),a high discharge peak power density(340 mW cm^(-2)) and a large specific capacity(775 mAh g^(-1) at 10 mA cm^(-2)) for the primary Zn-air battery, a small charge-discharge voltage gap and a high cycle-life(504 cycles at 10 mA cm^(-2) with 10 min per cycle) for the rechargeable Zn-air battery. In particular, the simple synthesis method is suitable for a large-scale production of this bifunctional material due to a green, cost effective and readily available process.

Lamellar-stacked cobalt-based nanopiles integrated with nitrogen/sulfur co-doped graphene as a bifunctional electrocatalyst for ultralong-term zinc-air batteries

Sluggish oxygen evolution reaction(OER)and oxygen reduction reaction(ORR)kinetics inevitably impede the practical performance of rechargeable zinc-air batteries.Thus,combing the structural designability of transition metal-based electrocatalysts with anionic regulation is highly desired.Herein,mesoporous lamellar-stacked cobalt-based nanopiles with surface-sulfurization modification are elaborately designed and integrated with N/S co-doped graphene to build a robust OER/ORR bifunctional electrocatalyst.The lamellar-stacking mode of mesoporous nanosheets with abundant channels accelerates gas-liquid mass transfer,and partial-sulfurization of cobalt-based matrix surface efficiently improves the intrinsic OER activity.Meanwhile,N/S co-doped graphene further reinforces the ORR active sites while providing a stable conductive skeleton.As expected,this composite electrocatalyst delivers considerable bifunctional activity and stability,with an OER overpotential of 323 m V at 10 m A cm^(-2)and high durability.When applied in zinc-air batteries,remarkable ultralong-term stability over 4000 cycles and a maximum power density of 150.1 m W cm^(-2)are achieved.This work provides new insight into structurecomposition synergistic design of rapid-kinetics OER/ORR bifunctional electrocatalyst for nextgeneration metal-air batteries.

Application of Fe-Ni-W Plated Film Electrode to Zinc-Air Battery

This study was aimed at the preparation of an electrode for Zinc–air battery, which had excellent catalytic activity by use of electroplating of alloys made of abundant metal, such as Fe and Ni. The oxygen overvoltage of the Fe-Ni-W alloy plated electrode was the smallest through the measurement. The elemental composition and the enlargement of the surface area were confirmed by SEM and EDX analysis. Involvement of Fe and W of Fe-Ni-W alloy plated electrode will be one factor for its high catalytic activity. Thus plated Fe-Ni-W alloy electrodes were compared with other Fe alloy plated elec-trodes considering to their cathode performance as Zinc-air battery. The catalytic activity of Fe-Ni-W plated electrode showed the best performance comparing to Fe-Ni alloy plated electrodes as cathode for Zinc-air battery. Also comparing to the platinum electrode which had been widely used as cathode in the field of Zinc-air battery, the Fe-based alloy plated electrode showed better performance as the electrodes considering to its oxygen evolution reaction.