Application Of Bifunctional Electrocatalysts Based On Cobalt-Based Selenide Catalysts In Zn-Air Batteries

In order to achieve the goal of carbon neutrality,it is becoming increasingly important to develop sustainable and efficient energy conversion and storage technologies.Among them,rechargeable Zn-air batteries have emerged as promising next generation electrochemical energy storage systems owing to their inherent safety,environmental friendliness,high theoretical energy density and cost-effectiveness.However,the sluggish kinetics of oxygen electrode reactions is a bottleneck for the development of energy efficiency and cycle life of rechargeable Zn-air batteries.In recent years,transition metal selenides have shown good tunable performance in various electrochemical applications.With numerous advantages such as highly conductive metal behaviour,low cost,tunable electronic structure and good catalytic activity,cobalt-based selenides are one of the most promising materials for bifunctional catalysts for rechargeable Zn-air batteries.Therefore,in this thesis,cobalt-based selenides are investigated and the catalysts are designed through interfacial engineering,morphological engineering and defect engineering perspectives as follows.（1）The following design is carried out to improve the problem of low active sites exposed to CoSe₂.A bifunctional catalyst of Co/CoSe₂@CNx has been successfully synthesized by high temperature calcination-selenisation using a 2D metal-organic framework as the precursor.Thanks to the spontaneous electron transfer at the interface of Co and CoSe₂,Co/CoSe₂@CNx exhibits excellent multifunctional electrocatalytic activity.conventional Zn-air batteries based on Co/CoSe₂@CNx catalysts have a high specific capacity(813.06 m Ah g_Zn^-1).In order to reduce the charging voltage of the batteries,a urea-assisted zinc-air batteries has been assembled based on the Co/CoSe₂@CNx catalyst.This is a narrower charge/discharge voltage gap than conventional zinc-air batteries and can be stable for up to 140 h.（2）To improve the OER activity of the catalyst,CoSe₂is reduced to a nonstoichiometric ratio of Co_0.85Se.A new Co_0.85Se@Co/N-C nanocomposite with a core-shell structure have been prepared using an in-situ growth-calcination strategy.Based on the unique beaded structure and the strong synergistic effect between Co_0.85Se and Co/N-C,the synthesized Co_0.85Se@Co/N-C exhibited excellent catalytic activity and stability towards ORR and OER.In alkaline media,a half-wave potential of Co_0.85Se@Co/N-C up to 0.82V and a low overpotential of 0.357 V at 10 m A cm^-2.Thus,Co_0.85Se@Co/N-C has a high bifunctional activity.Both liquid Zn-air batteries and flexible Zn-air batteries based on Co_0.85Se@Co/N-C catalysts have good cell performance.The liquid Zn-air batteries based on the Co_0.85Se@Co/N-C catalyst exhibited a power density of 122.92 m W cm^-2.And the flexible Zn-air batteries assembled on the basis of this catalyst exhibited good multiplicative performance.（3）We have doped Co_0.85Se with metal cations in order to further improve the bifunctional catalytic activity of the catalyst.Se and N co-doped porous carbon nanoplates loaded with Fe-Co_0.85Se are prepared by high-temperature calcination.the valence state of Co is enhanced by the introduction of Fe,and the catalytic activity is improved.The porous 2D nanoplates facilitate the exposure of more active sites and promote the transport of electrolyte and oxygen molecules.As a result,the Fe-Co_0.85Se/NSe C catalysts exhibited excellent ORR（0.83 V）and OER（0.353 V）activity as well as long cycle stability.Furthermore,zinc-air batteries using Fe-Co_0.85Se/NSe C catalysts as air electrodes exhibited high open-circuit voltage（1.51 V）,superior power density(163.1 m W cm^-2)and long-cycle stability.