Preparation Of Graphene Composites And The Application In Zinc-air Batteries

With the deepening of the energy crisis and the orderly promotion of the construction of an environment-friendly society,the design and development of green,efficient,safe and low-cost energy storage and transformation systems have become a research hotspot.In recent years,rechargeable zinc-air batteries have been gained extensive attention due to their low-cost,high safety,high specific energy and environmental friendliness.However,the kinetic pathways of the core reaction（oxygen reduction reaction and oxygen evolution reaction）of the zinc-air batteries are very slow,and the existing noble metals and their oxides electrocatalysts（Pt/Ir/Ru）have a high cost,shortage of resources and poor stability,and often show only single-function electrocatalytic performance,which seriously limits the development in the zinc-air batteries.Therefore,the design of electrocatalysts with efficient,low-cost,resource-rich and bifunctional is the key to solve the above problems in zinc-air batteries.1.The preparation of surface-improved thin-layer carbon nitride/graphene oxide composite for zinc-air batteries.In this paper,firstly,graphene oxide（RGO）was synthesized by the improvedHummers method.Then,the improved carbon nitride materials were synthesized by a certain proportion of cobalt salt with melamine.Finally,the above-improved carbon nitride materials and graphene oxide were used to prepare the surface-modified carbon nitride and graphene oxide composites by hydrothermal reaction.Using modern techniques and electrochemical research methods such as cyclic voltammetry and linear scanning voltammetry,it was found that the obtained composite of cobalt oxide/carbon nitride/reducing graphene oxide（Co O_x/PCN/RGO）was a thin-layer structure with a large surface area,which greatly improved the pure carbon nitride（g-C₃N₄）properties and exhibited excellent bifunctional catalytic activity with an overpotential difference of 0.95 V,which was much smaller than that of the mixture of platinum carbon and iridium carbon（Pt/C+Ir/C:1.0 V）and also exhibited superior power density(120 m W cm^-2)and charge/discharge cycle stability（102 h）in self-made zinc-air batteries.2.Preparation of cobalt/cobalt oxide/nitrogen-doped graphene oxide bifunctional electrocatalysts for zinc-air batteries.Next,cobalt/cobalt oxide/nitrogen-doped reduced graphene oxide（Co/Co O/NRGO） composites were synthesized using cobalt nitrate,phenanthroline and graphene oxide as raw materials by simple physical stirring.Among them,graphene oxide as a carbon source and phenanthroline serves as both chelating reagent and available nitrogen source.In addition,the various characterization methods further show that Co/Co O/NRGO composite is a porous thin-layer structure.The ideal synergistic effect of Co,Co O and NRGO make the as-prepared Co/Co O/NRGO-2 composites exhibit excellent bifunctional catalytic performance with a half-wave potential of 0.81 V for oxygen reduction reaction（ORR）and 1.79 V for oxygen evolution reaction（OER）at 10 m A cm^-2.As cathode materials for air electrodes,exhibit superior power density(270 m W cm^-2)and charge/discharge cycle life（230 h）over the Pt/C+Ir/C mixture.3.The preparation of novel metal sulfide/graphene bifunctional electrocatalyst with a"dual-carbon"structure for zinc-air batteries.Finally,metal sulfides/graphene composites（Co₉S₈/GN）with the unique dual-carbon structures were prepared using ZIF-67 as precursor,thioacetamide and graphene as sulfur source and a carbon source.A series of characterization techniques and electrochemical methods confirm that this structure can establish fast electron transfer channels to overcome the difficult dispersion of metal sulfides and the reaccumulation of graphene.At the same time,the effects of reaction temperature and the amount of graphene on the electrocatalytic performance of the Co₉S₈/GN composites were also investigated.As a result,it was found that the Co₉S₈/GN-0.2（180℃）composites（ΔE:0.88 V）exhibited better bifunctional catalytic activity than that of Pt/C+Ir/C mixture（ΔE:0.91 V）.In the assembled zinc-air batteries,the Co₉S₈/GN-0.2（180℃）composites(186 m W cm^-2)exhibited a power density beyond Pt/C+Ir/C mixture(118 m W cm^-2).In charge-discharge stability,it could charge and discharge up to 2000 cycles（Pt/C+Ir/C:50 cycles）,which is expected to be an alternative to Pt/C+Ir/C catalysts.