| The increasing scarcity of oil resources and greenhouse gas emissions caused the intensification of global warming,which made the development and utilization of new energy and technology highly urgent subjects.Nonaqueous Li-air batteries showed great potential for various applications due to its extremely high capacity density.The air electrode is the main place for the battery reactions,which determines the efficiency of the electrode reactions(ORR and OER),and influences the discharge/charge performance and cyclic stability.Therefore,the development of air electrode is an urgent project for the practical application of the nonaqueous Li-air batteries.Previous reports showed that the introduction of proper electrocatalysts in the cathode of the batteries can effectively reduce the polarization of the electrochemical reactions and improve the discharge/charge performance.Graphene showed several advantages due to the novel structure and morphology,which can be used not only as a support for electrocatalyst,but also an ideal model system to study the activity,stability and catalytic mechanism,ect.(1)The mixture of graphite oxide and solid sulfur source or nitrogen/sulfur source were heated by microwave irradiation,and during this process,the graphene can be reduced and doped in the same time.The results showed that the doped graphene can be prepared by microwave heating in a very short time and after doping,the(002)diffraction peak of graphene was shifted and the intensity of peak was increased as the amount of nitrogen and sulfur precursor increased.The morphology of graphene and doped-graphene showed no obvious change,which indicated that using the solid-state reaction can maintain the morphology of few-layer graphene,resulting in high surface area.When the mass ratio of graphite oxide and thiourea was 3: 1,the intensity ratio of D-band and G-band from Raman spectrum was 0.99,which was larger than that of undoped graphene(0.81).And the best oxygen reduction catalytic activity was obtained for this sample.Combining the microwave heating and the chemical reduction method,the Pd W alloy can be reduced in a short time and supported on doped or undoped graphene.The results showed that when the atomic ratio of Pd and W was 3: 1,as well as the loading amount of Pd W alloy is 20% on graphene,the highest catalytic performance was achieved.(2)Pd3W-SG electrocatalysts were prepared by the microwave-heating and chemical reduction method.The results showed that: 1.There was no impurity phase observed in Pd W alloy,the crystallinity degree of the alloy was high,and moreover,the alloy particles were uniformly dispersed on the surface of sulfur-doped graphene;2.From the cyclic voltammetry(CV)tests for ORR in alkaline electrolyte,Pd3W-SG showed the most positive onset potential of-0.02 V,while for other samples were Pd-SG(-0.12 V),Pd-G(-0.12 V),SG(-0.19 V)and G(-0.21 V),respectively,which indicated that the doped graphene supported Pd W alloy electrocatalyst has higher ORR catalytic activity;3.The Rotating disk electrode(RDE)and rotation ring disk electrode(RRDE)tests,the Pd W-SG electrocatalyst showed that the transfer electron number was close to 4,which was comparable to commercial platinum carbon(Pt/C)catalyst;5.From the anti-poisioning tests of Pd3W-SG and commercial Pt/C,the current density of Pd3W-SG was only changed for 0.04706 m A/cm2,while the commercial Pt/C was changed for 0.09094 m A/cm2 after adding the methanol;6.Form the stability tests of Pd3W-SG and commercial Pt/C,the peak current density of Pd3W-SG was only changed for 0.01854 m A/cm2,while the Pt/C was changed for 0.07669 m A/cm2;7.For Li-air battery performance test,when the discharge current density was 100 m A/g,Pd3W-SG catalyst as a positive electrode showed a higher specific capacity of 5660.8 m Ah/g and a longer cycle life.(3)Pd3W-NSG electrocatalysts were prepared by the the microwave-heating and chemical reduction method.The results showed that: 1.There was no impurity phase observed in Pd W alloy,the crystallinity degree of the alloy was high,and moreover,the alloy particles were uniformly dispersed on the surface of nitrogen/sulfur-doped graphene;2.The particle sizes of Pd-G,Pd-NSG and Pd3W-NSG samples were 14.33 nm,32.8 nm and 20.3 nm,respectively.Nanoparticles with smaller size had a larger specific surface area,which can effectively improve the electrochemical swface area of the catalyst to improve catalytic activity;3.From the cyclic voltammatry(CV)tests for ORR in alkaline electrolyte,Pd3W-NSG showed the most positive onset potential of-0.014 V,while for other samples were Pd-NSG(-0.126 V),Pd-G(-0.143 V),NSG(-0.187 V)and G(-0.204 V),respectively,which indicated that the doped graphene supported Pd W alloy electrocatalyst has higher ORR catalytic activity;4.From the anti-poisioning tests of Pd3W-NSG and commercial Pt/C,the current density of Pd3W-NSG was only changed for 0.02502 m A/cm2,while the commercial Pt/C was changed for 0.09094 m A/cm2 after adding the methanol;5.From the stability test of Pd3W-NSG and commercial Pt/C,the peak current density of Pd3W-NSG was only changed for 0.01819 m A/cm2,while the Pt/C was changed for 0.07669 m A/cm2;6.For Li-air battery performance test,when the discharge current density was 100 m A/g,Pd3W-NSG catalyst as a positive electrode showed a longer specific capacity of 7441.1 m Ah/g and a higher cycle life;7.For the Scanning Electrochemical Microscopy(SECM)test,Pd3W-NSG exhibited the highest catalytic activity. |
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