A Porous Nano-Micro-Composite as a High-Performance Bi-Functional Air Electrode with Remarkable Stability for Rechargeable Zinc-Air Batteries

The development of bi-functional electrocatalyst with high catalytic activity and stable performance for both oxygen evolution/reduction reactions(OER/ORR)in aqueous alkaline solution is key to realize practical application of zinc-air batteries(ZABs).In this study,we reported a new porous nano-micro-composite as a bifunctional electrocatalyst for ZABs,devised by the in situ growth of metal-organic framework(MOF)nanocrystals onto the micrometersized Ba0.5Sr0.5Co0.8Fe0.2O3(BSCF)perovskite oxide.Upon carbonization,MOF was converted to porous nitrogen-doped carbon nanocages and ultrafine cobalt oxides and CoN4 nanoparticles dispersing inside the carbon nanocages,which further anchored on the surface of BSCF oxide.We homogeneously dispersed BSCF perovskite particles in the surfactant;subsequently,ZIF-67 nanocrystals were grown onto the BSCF particles.In this way,leaching of metallic or organic species in MOFs and the aggregation of BSCF were effectively suppressed,thus maximizing the number of active sites for improving OER.The BSCF in turn acted as catalyst to promote the graphitization of carbon during pyrolysis,as well as to optimize the transition metal-tocarbon ratio,thus enhancing the ORR catalytic activity.A ZAB fabricated from such air electrode showed outstanding performance with a potential gap of only 0.83 V at 5 mA cm-2 for OER/ORR.Notably,no obvious performance degradation was observed for the continuous charge-discharge operation for 1800 cycles over an extended period of 300 h.

Recent Advances in the Understanding of the Surface Reconstruction of Oxygen Evolution Electrocatalysts and Materials Development

The electrochemical oxygen evolution reaction(OER)plays an important role in many clean electrochemical energy storage and conversion systems,such as electrochemical water splitting,rechargeable metal–air batteries,and electrochemical CO_(2) reduction.However,the OER involves a complex four-electron process and suffers from intrinsically sluggish kinetics,which greatly impairs the efficiency of electrochemical systems.In addition,state-of-the-art RuO2-based OER electrocatalysts are too expensive and scarce for practical applications.The development of highly active,cost-effective,and durable electrocatalysts that can improve OER performance(activity and durability)is of significant importance in realizing the widespread application of these advanced technologies.To date,considerable progress has been made in the development of alternative,noble metal-free OER electrocatalysts.Among these alternative catalysts,transition metal compounds have received particular attention and have shown activities comparable to or even higher than those of their precious metal counterparts.In contrast to many other electrocatalysts,such as carbon-based materials,transition metal compounds often exhibit a surface reconstruction phenomenon that is accompanied by the transformation of valence states during electrochemical OER processes.This surface reconstruction results in changes to the true active sites and an improvement or reduction in OER catalytic performance.Therefore,understanding the self-reconstruction process and precisely identifying the true active sites on electrocatalyst surfaces will help us to finely tune the properties and activities of OER catalysts.This review provides a comprehensive summary of recent progress made in understanding the surface reconstruction phenomena of various transition metal-based OER electrocatalysts,focusing on uncovering the correlations among structure,surface reconstruction and intrinsic activity.Recent advances in OER electrocatalysts that exhibit a surface self-reconstruction capability are also discussed.We identify possible challenges and perspectives for the development of OER electrocatalysts based on surface reconstruction.We hope this review will provide readers with some guidance on the rational design of catalysts for various electrochemical reactions.

Recent Progress of Graphene Fiber/Fabric Supercapacitors:From Building Block Architecture,Fiber Assembly,and Fabric Construction to Wearable Applications

High-performance fiber-shaped power sources are anticipated to considerably contribute to the continuous development of smart wearable devices.As one-/two-dimensional(1D/2D)frameworks constructed from graphene sheets,graphene fibers and fabrics inherit the merits of graphene,including its lightweight nature,high electrical conductivity,and exceptional mechanical strength.The as-fabricated graphene fiber/fabric flexible supercapacitor(FSC)is,therefore,regarded as a promis-ing candidate for next-generation wearable energy storage devices owing to its high energy/power density,adequate safety,satisfactory flexibility,and extended cycle life.The gap between practical applications and experimental demonstrations of FSC is drastically reduced as a result of technological advancements.To this end,herein,recent advancements of FSCs in fiber element regulation,fiber/fabric construction,and practical applications are methodically reviewed and a forecast of their growth is presented.