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.

Engineering Two-Phase Bifunctional Oxygen Electrocatalysts with Tunable and Synergetic Components for Flexible Zn-Air Batteries

Metal-air batteries,like Zn-air batteries(ZABs)are usually suffered from low energy conversion efficiency and poor cyclability caused by the sluggish OER and ORR at the air cathode.Herein,a novel bimetallic Co/CoFe nanomaterial supported on nanoflower-like N-doped graphitic carbon(NC)was prepared through a strategy of coordination construction-cation exchange-pyrolysis and used as a highly efficient bifunctional oxygen electrocatalyst.Experimental characterizations and density functional theory calculations reveal the formation of Co/CoFe heterostructure and synergistic effect between metal layer and NC support,leading to improved electric conductivity,accelerated reaction kinetics,and optimized adsorption energy for intermediates of ORR and OER.The Co/CoFe@NC exhibits high bifunctional activities with a remarkably small potential gap of 0.70 V between the half-wave potential(E_(1/2))of ORR and the potential at 10 mA cm^(-2)(E_(j=10))of OER.The aqueous ZAB constructed using this air electrode exhibits a slight voltage loss of only 60 mV after 550-cycle test(360 h,15 days).A sodium polyacrylate(PANa)-based hydrogel electrolyte was synthesized with strong water-retention capability and high ionic conductivity.The quasi-solid-state ZAB by integrating the Co/CoFe@NC air electrode and PANa hydrogel electrolyte demonstrates excellent mechanical stability and cyclability under different bending states.

Paired formate and H_(2) productions via efficient bifunctional Ni-Mo nitride nanowire electrocatalysts

Electrocatalytic water splitting provides a potentially sustainable approach for hydrogen production,but is typically restrained by kinetically slow anodic oxygen evolution reaction(OER)which is of lesser value.Here,free-standing,hetero-structured Ni_(3)N-Ni_(0.2)Mo_(0.8)N nanowire arrays are prepared on carbon cloth(CC)electrodes for hydrogen evolution reaction(HER)and glycerol oxidation reaction(GOR)to formate with a remarkably high Faradaic efficiency of 96%.A two-electrode electrolyzer for GOR-assisted hydrogen production operates with a current density of 10 mA cm^(-2)at an applied cell voltage of 1.40 V,220 mV lower than for alkaline water splitting.In-situ Raman measurements identify Ni(Ⅲ)as the active form of the catalyst for GOR rather than Ni(IV)and in-situ Fourier transform infrared(FTIR)spectroscopy measurements reveal pathways for GOR to formate.From density functional theory(DFT)calculations,the Ni_(3)N-Ni_(0.2)Mo_(0.8)N heterostructure is beneficial for optimizing adsorption energies of reagents and intermediates and for promoting HER and GOR activities by charge redistribution across the heterointerface.The same electrode also catalyzes conversion of ethylene glycol from polyethylene terephthalate(PET)plastic hydrolysate into formate.The combined results show that electrolytic H_(2) and formate production from alkaline glycerol and ethylene glycol solutions provide a promising strategy as a cost-effective energy supply.

High-capacity Bi_(2)O_(3) anode for 2.4 V neutral aqueous sodium-ion battery-supercapacitor hybrid device through phase conversion mechanism

Aqueous battery-supercapacitor hybrid devices(BSHs)are of great importance to enrich electrochemical energy storage systems with both high energy and power densities.However,further improvement of BSHs in aqueous electrolytes is greatly hampered by operating voltage and capacity limits.Different from the conventional intercalation/de-intercalation mechanism,Bi_(2)O_(3) implements charge storage by a reversible phase conversion mechanism.Herein,taking Bi_(2)O_(3) electrode with wide potential window(from-1.2 to 1 V vs.saturated calomel electrode)and high capacity as battery-type anode,we propose that the overall performance of aqueous BSHs can be greatly upgraded under neutral condition.By paring with stable layer-structuredδ-MnO_(2) cathode,a sodium-ion Bi_(2)O_(3)//MnO_(2) BSH with an ultrahigh voltage of 2.4 V in neutral sodium sulfate electrolyte is developed for the first time.This hybrid device exhibits high capacity(~215 C g^(-1) at 1 mA cm^(-2)),relatively long lifespan(~77.2%capacity retention after 1500 cycles),remarkable energy density(71.7 Wh kg^(-1)@400.5 W kg^(-1))and power density(3204.3 W kg^(-1)@18.8 Wh kg^(-1)).Electrochemical measurements combining a set of spectroscopic techniques reveal the reversible phase conversion between bismuth oxide and metallic bismuth(Bi_(2)O_(3)?Bi0)through Bi^(2+) transition phase in neutral sodium sulfate solution,which can deliver multielectron transfer up to 6,leading to the high-energy BSHs.Our work sheds light on the feasibility of using Bi_(2)O_(3) electrode under neutral condition to address the issue of narrow voltage and low capacity for aqueous BSHs.

A novel heterogeneous Co(Ⅱ)-Fenton-like catalyst for efficient photodegradation by visible light over extended pH

A novel Co^Ⅱ-Fenton-like heterogeneous catalyst,(H3O)2[Co^Ⅱ(phen)(H2O)2]2[Mo^Ⅵ5O15(PO4)2]·4H2O (phen=1,10-phenanthroline,C12N2H8)(1),is synthesized and utilized for photocatalytic degradation of organic dyes and antibiotic in a wide range of pH.The experimental results show that 1 acting as the Fenton-like catalyst with H2O2 exhibits remarkable activity at pH 3–9 under vis-light irradiation and is merited with excellent recyclability and reusability.A variety of analytical methods,including in-situ electron paramagnetic resonance (EPR) spectroscopy and density functional theory (DFT) calculations,are applied to study the mechanism on generation of·OH and O2^·-radicals for photocatalytic degradation.It suggests that,unlike the classical Fenton process involving the redox transformation of the central cations,the generation of·OH and O2^·-radicals is associated with the substitution of the coordinating water molecules at Co(Ⅱ) by H2O2 and/or OOH^-,followed by the light-driven O–O bond cleavage and dissociation.The outcomes of this study are striking which overcome the obstacles in the classical Fe^Ⅱ-Fenton process,including the slow redox transformation between Fe(Ⅱ) and Fe(Ⅲ) and the production of massive iron precipitates especially at elevated pH.It opens up new avenues for the development of the high-performance Fenton(-like) catalysts for photocatalytic degradation over extended pH and provides new insight into the related process.

Temperature-controlled fabrication of Co-Fe-based nanoframes for efficient oxygen evolution

Transition metal phosphides(TMPs)have emerged as promising electrocatalysts to enhance the slow kinetic process of oxygen evolution reaction(OER).Framelike hollow nanostructures(nanoframes,NFs)provide the open structure with more accessible active sites and sufficient channels into the interior volume.Here,we report the fabrication of bimetallic Co-Fe phosphide NFs(Co-Fe-P NFs)via an intriguing temperature-controlled strategy for the preparation of precursors followed by phosphidation.The precursors,Co-Fe Prussian blue analogues(Co-Fe PBAs)are prepared by a precipitation method with Co^(2+)and[Fe(CN)_(6)]^(3−),which experience a structural conversion from nanocubes to NFs by increasing the aging temperature from 5 to 35℃.The experimental results indicate that this conversion is attributable to the preferentially epitaxial growth on the edges and corners of nanocubes,triggered by intramolecular electron transfer at an elevated aging temperature.The as-prepared Co-Fe-P NFs catalyst shows remarkable catalytic activity toward OER with a low overpotential of 276 mV to obtain a current density of 10 mA cm^(−2),which is superior to the reference samples(Co-Fe-P nanocubes)and most of the recently reported TMPs-based electrocatalysts.The synthetic strategy can be extended to fabricate Co-Fe dichalcogenide NFs,thereby holding a great promise for the broad applications in energy storage and conversion systems.

Upcycling PET in parallel with energy-saving H_(2)production via bifunctional nickel-cobalt nitride nanosheets

We describe here an electro-reforming strategy to upcycle polyethylene terephthalate(PET)waste with simultaneous hydrogen production by a bifunctional nickel-cobalt nitride nanosheets electrocatalyst.PET plastics are digested in alkaline solution giving an electrochemically active monomer ethylene glycol(EG).The introduction of Co in Co-Ni3N/carbon cloth(CC)promotes the redox behavior of Ni2+/Ni3+,which is beneficial for EG oxidation at an ultra-low potential(1.15 V vs.reversible hydrogen electrode(RHE))and breaks through the limitation of high catalytic potentials of simple Ni-based electrocatalysts(1.30 V).In PET hydrolysate with Co-Ni3N/CC couples,an integrated EG oxidation-hydrogen production system achieves a current density of 50 mA·cm^(−2)at a cell voltage of 1.46 V,which is 370 mV lower than the conventional water splitting.The in-situ Raman and Fourier transform infrared(FTIR)spectroscopies and density functional theory(DFT)calculations identify the catalytic mechanism and point to advantages of heterostructure engineering in optimizing adsorption energies and promoting catalytic activities for EG oxidation.

Engineering iron-group bimetallic nanotubes as efficient bifunctional oxygen electrocatalysts for flexible Zn–air batteries

Air cathode performance is essential for rechargeable zinc–air batteries(ZABs).In this study,we develop a self-templated synthesis technique for fabricating bimetallic alloys(FeNi_(3)),bimetallic nitrides(FeNi_(3)N)and heterostructured FeNi_(3)/FeNi_(3)N hollow nanotubes.Owing to its structural and compositional advantages,FeNi_(3)/FeNi_(3)N exhibits remarkable bifunctional oxygen electrocatalytic performance with an extremely small potential gap of 0.68​V between the oxygen evolution reaction(OER)and oxygen reduction reaction(ORR).Theoretical calculations reveal reduced Gibbs free energy for the rate-limiting O–O bond formation during OER due to the self-adaptive surface reconfiguration,which induces a synergistic effect between Fe(Ni)OOH developed in situ on the surface and the inner FeNi_(3)/FeNi_(3)N.ZAB fabricated using the FeNi_(3)/FeNi_(3)N catalyst shows high power density,small charge/discharge voltage gap and excellent cycling stability.In addition to its excellent battery performance,the corresponding quasi-solid-state ZAB shows robust flexibility and integrability.The synthesis method is extended to prepare a CoFe/CoFeN oxygen electrocatalyst,demonstrating its applicability to other iron-group elements.

N-doped graphitic carbon encapsulating cobalt nanoparticles derived from novel metal–organic frameworks for electrocatalytic oxygen evolution reaction

Nitrogen-doped carbon catalysts with hierarchical porous structure are promising oxygen evolution reaction(OER)catalysts due to the faster mass transfer and better charge carrying ability.Herein,an exquisite high nitrogen-containing ligand was designed and readily synthesized from the low-cost biomolecule adenine.Accordingly,three new MOFs(TJU-103,TJU-104 and TJU-105)were prepared using the Co(II)or Mn(II)ions as metal nodes.Through rationally controlling pyrolysis condition,in virtue of the high nitrogen content in well-defined periodic structure of the pristine MOFs,TJU-104–900 among the derived MOFs with hierarchical porous structure,i.e.,N-doped graphitic carbon encapsulating homogeneously distributed cobalt nanoparticles,could be conveniently obtained.Thanks to the synergistic effect of the hierarchical structure and well dispersed active components(i.e.,C=O,Co–Nx,graphitic C and N,pyridinic N),it could exhibit an overpotential of 280 mV@10mA/cm^(2)on carbon cloth for OER activity.This work provides the inspiration for fabrication of nitrogen-doped carbon/metal electrocatalysts from cost-effective and abundant biomolecules,which is promising for practical OER application.

Fabrication of complex,3D,branched hollow carbonaceous structures and their applications for supercapacitors

A unique“integrated hard-templating strategy”is described for facile synthesis of a carbonaceous material with a novel three-dimensional(3 D)branched hollow architecture.A set of steps,including template formation,surface coating and template removal,all occur in a spontaneous and orderly manner in the one-pot hydrothermal process.Investigations on structural evolution during the process reveal that pre-synthesized zeolitic imidazolate framework-8(ZIF-8)nanoparticles are first dissociated and then self-assembled into 3 D branched superstructures of ZnO as templates.Initial self-assembly is followed by coating of the glucose-derived carbonaceous materials and etching of interior ZnO by organic acids released in situ by hydrolysis of glucose.The 3 D-branched hollow architecture is shown to greatly enhance supercapacitor performance.The research described here provides guidance into the development of strategies for complex hollow carbonaceous architectures for a variety of potential applications.