Abstract

Recent fruitful studies on rechargeable zinc-air battery have led to emergence of various bifunctional oxygen electrocatalysts, especially metal-based materials. However, their electrocatalytic configuration and evolution pathway during battery operation are rarely spotlighted. Herein, to depict the underlying behaviors, a concept named dynamic electrocatalyst is proposed. By selecting a bimetal nitride as representation, a current-driven “shell-bulk” configuration is visualized via time-resolved X-ray and electron spectroscopy analyses. A dynamic picture sketching the generation and maturation of nanoscale oxyhydroxide shell is presented, and periodic valence swings of performance-dominant element are observed. Upon maturation, zinc-air battery experiences a near two-fold enlargement in power density to 234 mW cm−2, a gradual narrowing of voltage gap to 0.85 V at 30 mA cm−2, followed by stable cycling for hundreds of hours. The revealed configuration can serve as the basis to construct future blueprints for metal-based electrocatalysts, and push zinc-air battery toward practical application.

Highlights

  • Recent fruitful studies on rechargeable zinc-air battery have led to emergence of various bifunctional oxygen electrocatalysts, especially metal-based materials

  • Increasing reports on spectroelectrochemistry toward the half-reactions, that is, oxygen reduction (ORR) or evolution reactions (OER), in threeelectrode systems suggest that an unexplored chasm is at work

  • The latest attention has been drawn toward ORR electrocatalysts, in which an alteration was demonstrated[26]. These studies expose an often-overlooked fact that directly assuming the native state of metal-based electrocatalyst as the active representation in Zn–air battery operation may lead to false correlation between material characteristics and performance, albeit the undeniable role of their native properties

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Summary

Introduction

Recent fruitful studies on rechargeable zinc-air battery have led to emergence of various bifunctional oxygen electrocatalysts, especially metal-based materials. Hu and coworkers reported both nickel iron diselenide and nickel phosphide as efficient and long-lasting OER electrocatalysts, but neither of them is static stereotype under oxidation voltage in alkaline electrolyte[21,22] This contradiction was resolved with observations of surficial phase transformation from these compounds to their derived oxides or oxyhydroxides, which aligns with the results on metal oxides, sulfides, and nitrides[19,23,24,25]. Thereafter, currentdriven valence swings of surficial Co are observed, while Fe at the surface and the bulk regions remains relatively inert As a result, it is reflected electrochemically by an increase in power density up to 234 mW cm−2, shrink of voltage gap to 0.85 V at 30 mA cm–2, and long-lasting stability for over 300 h in Zn–air battery

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