Construction And Electrocatalytic Performances Of Iron-based Species Coupled Nitrogen-doped Graphene Nanosheets

Due to the limited resource of carbon-based fuels such as coal,natural gas and petroleum,the development of novel approaches to generate energy with high efficiency,stability,cleanness and low cost has received tremendous attention.Zinc–air batteries have been considered as promising substitutes for use in the next-generation energy conversion system because of their high theoretical energy density,environmental friendliness and low cost.Electrocatalytic oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?are two key constituent parts for zinc–air batteries,and critical to their practical performances.However,both reactions need the assistance of electrocatalysts to conquer activation energy barriers and accelerate reaction processes.Currently,although precious metals?Pt,Ir and Ru?are the most efficient oxygen electrocatalysts,their high costs and poor durability severely limit their large-scale applications.Therefore,there is an urgent demand to develop highly efficient,low-cost as well as earth-abundant alternatives.In this thesis,in order to accelerate the reaction kinetics for ORR and OER,the N-doped carbon nanosheets are first designed and synthesized.By introducing iron-based species to increase the number of active sites,iron-based species coupled nitrogen-doped graphene nanosheets are fabricated.Through the synergistic effect between iron-based species and N-doped carbon matrix,the electrocatalytic activity of the catalyst is enhanced.This thesis mainly focuses on the following:?1?The N-doped graphitic nanosheet is synthesized via a facile pyrolysis route,in which the graphitic carbon nitride is used as the main nitrogen source and template,and graphene oxide served as the carbon matrix.After adding sodium alginate,the aerogel precursor is obtained.Through high-temperature pyrolysis and ammonia activation,the resulting NGNs catalyst exhibits a high content of pyridinic and graphitic N with desirable ORR activity.?2?Based on the above work,through using FeOOH as the iron source,the N-doped graphene wrapped Fe₃C/Fe₂O₃ bifunctional catalyst?Fe₃C/Fe₂O₃@NGNs?has been synthesized for ORR and OER.The existence of Fe₃C/Fe₂O₃ heterostructure allows the catalyst to display an outstanding ORR activity with the half-wave potential of0.86 V and desirable OER activity with the potential of 1.69 V at a current density of10 mA cm^-22 in alkaline media.At the same time,the zinc–air battery assembled with the Fe₃C/Fe₂O₃@NGNs cathode presents a better charging–discharging cycling performance than that of the battery with Pt/C+IrO₂ mixed catalysts.?3?With the further improvement of the synthetic method based on previous work,amorphous Cu and Fe dual-metal clusters encapsulated in N-doped graphene is successfully prepared via a“short-term vacuum high-temperature pyrolysis”synthesis route.This method not only can achieve the non-crystallographic structure of Cu and Fe species,but also does not require a long-term thermal process and the use of inert gases.Remarkably,owing to its bimetallic active sites,the resulting Cu/Fe-NG catalyst exhibits outstanding electrocatalytic ORR performances,manifesting in a positive half-wave potential?0.88 V?,low hydrogen peroxide yield and excellent long-term durability.Density functional theory?DFT?calculations further reveal that Fe-N₄ reactive center coupling with Cu-N_x species favors the adsorption of O₂molecule and therefore accelerates the ORR process.The liquid zinc–air battery assembled with the Cu/Fe-NG cathode exhibits a high open circuit voltage of 1.53 V,large energy density of 164.2 mW cm^–2 and specific capacity of 727.5 mAh g^–1.Moreover,the Cu/Fe-NG also has desirable performances even in quasi-solid-state Zn–air batteries.