Preparation Of Graphene-based Oxygen Electrocatalysts And Application In Zn-air Batteries

Rechargeable Zn-air batteries（ZABs）have attracted great attentions due to the advantages of high energy density（1086 Wh/kg）,abundant reserves,low cost and intrinsic safety.The oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）occurred in cathode play pivotal roles in the discharging and charging process of rechargeable ZABs.Nevertheless,both reactions are multi-step electron transfer processes with high energy barriers and sluggish kinetics,thus severely jeopardizing the actual efficiency of rechargeable ZABs.Currently,commercial Pt/C and Ir O₂ are considered to be the start-of-the-art ORR and OER electrocatalysts,but they cannot meet the bi-functional activity requirements simultaneously.In addition,the shortcomings of scarcity,high price and poor durability inevitably suppress the large-scale application of ZABs.Therefore,it is imperative to develop high-efficient,stable and cheap noble-metal-free bifunctional electrocatalysts.During my doctoral study,I successfully prepared a variety of bifunctional oxygen electrocatalysts using graphene and its derivatives as carbon substrates and explored their actual performance in ZABs.The specific works as follows:（1）In order to prepare efficient ORR electrocatalysts,a hybrid heterostructure of N-doped graphene and carbon nanotubes at the pyrolysis temperature of 900°C（N-G/CNTs-900）was manufactured by a hydrothermal reduction and subsequent-pyrolysis strategy,which exhibits a better ORR activity(E_1/2=0.838 V)than most of reported carbon-based ORR electrocatalysts,and comparable to that of Pt/C.Thus-fabricated N-G/CNTs-900 features hierarchically porous architecture,high specific surface area,abundant defects,and high active N content,and what’s more,it also affords active sites for O₂ activation and subsequent 4e^-reduction processes due to the hetero-interlayer charge transfer and the electron accumulations of the carbon atoms on the surface layer by theoretical calculations.Compared with commercial Pt/C,N-G/CNTs-900-based ZABs presents a higher peak power density（133.60 m W/cm²）and a larger specific capacity(707 m Ah/g _Zn).（2）For preparing efficient bifunctional oxygen electrocatalysts,the edge-structure-abundant N-doped graphene nanoribbons（N-GNRs）were firstly selected as the substrate material,and the Fe Co nanoparticles coated with carbon layers on N-GNRs catalyst（Fe Co@C/N-GNRs）was successfully prepared by in-situ growth and subsequent pyrolysis strategy.Physical characterizations prove the structure that Fe Co nanoparticles coated with carbon layers on N-GNRs,and the strong interaction between Fe Co nanoparticles and carbon layer is conducive to the enhancement of catalytic activity,and the characteristics of carbon shell-coated nanoparticles in turn effectively improve the stability of the catalyst.Electrochemical tests show that Fe Co@C/N-GNRs possess excellent electrocatalytic performance(ORR:E_1/2=0.830 V;OER:E₁₀=1.530V;ΔE=0.700 V).The bifunctional activity for Fe Co@C/N-GNRs is not only superior to that of benchmark Pt/C+Ir O₂,but also better than most reported bifunctional oxygen electrocatalysts.The liquid and flexible ZABs based on Fe Co@C/N-GNRs catalyst show higher peak power densities（165.83 m W/cm²,53.92 m W/cm²）and better stability than Pt/C+Ir O₂-based ZABs.（3）In order to further improve the catalytic activity,the Fe-mel-ZIF-8 as a precursor was firstly prepared by in situ growth of Fe²⁺,Zn²⁺,melamine and 2-methylimidazole.Using obtained Fe-mel-ZIF-8 as the precursor,the Fe-containing bamboo-like CNTs bridged-with N-GNRs（Fe@BCNTs/N-GNRs）was successfully prepared through simple mechanical grinding and pyrolysis approach.Physical tests show that Fe@BCNTs/N-GNRs is a typical ferriferous bamboo-like carbon nanotube bridged graphene nanoribbons structure.The intruduction of Fe@BCNTs not only effectively improves the defect degree,electrochemical active area and conductivity,but also provides Fe-N_x sites with high ORR activity.Further electrochemical characterizations manifest that the Fe@BCNTs/N-GNRs catalyst has great durability and outstanding ORR/OER activity.The gap between ORR half-wave potential and OER potential at 10 m A/cm² current density is only 0.690 V,which is not only better than the benchmark Pt/C+Ir O₂,but also surpasses most reported bifunctional oxygen electrocatalysts.The assembly of Fe@BCNTs/N-GNRs catalyst into liquid and flexible ZABs also display high catalytic activity（189.19 m W/cm²,57.93 m W/cm²）and great stability.（4）Dual-phase heterointerface electrocatalysts constructed by oxygen reduction reaction（ORR）-and oxygen evolution reaction（OER）-active elements exhibit excellent bifunctional activity and long-term durability due to the abundant interface exposure and synergistic catalytic effect.we successfully fabricated a N-doped graphene nanoribbons and Co O（N-GNRs/Co O）nanocomposite as an efficient bifunctional oxygen electrocatalyst by a simple two-step strategy using prepared Co（OH）₂ lamella as Co O precursor.Density functional theory calculations and experimental characterizations prove that the formation of interface between N-GNRs and Co O can induce local charge redistribution,contributing to the improvement of catalytic activity and stability.The optimal N-GNRs/Co O catalyst possesses hierarchically porous architectures and presents satisfied bifunctional activities with a potential gap of 0.729V between the potential at 10 m A/cm² for OER and the half?wave potential for ORR,outperforming Pt/C+Ir O₂ and the majority of noble-metal-free bifunctional catalysts.Liquid and solid-state rechargeable ZABs assembled with N-GNRs/Co O as the cathode display satisfied peak power density and fantastic cycle stability,superior to that of benchmark Pt/C+Ir O₂ catalyst.（5）For purpose of further improving the bifunctional activity of the catalyst,N-doped graphene nanoribbons,Co O,and Co S nanocomposites（N-GNRs/Co O/Co S）were successfully prepared by chemical vapor deposition using thiourea as a precursor.Physical characterizations prove the successful introduction of S and the formation of the heterogeneous interface of Co O/Co S.Electrochemical tests show that the bifunctional oxygen electrocatalytic activity of N-GNRs/Co O/Co S was significantly enhanced compared with N-GNRs/Co O.The gap between ORR half-wave potential and OER potential at 10 m A/cm² current density is only 0.687 V,which was not only better than the benchmark Pt/C+Ir O₂（0.752 V）,but also outperforming recently reported bifunctional electrocatalysts.Moreover,N-GNRs/Co O/Co S based liquid and flexible ZABs exhibit high catalytic activity（158.44 m W/cm²,63.66 m W/cm²）,bending resistance and excellent stability.