Preparation And Application Of Transition Metal Compound/Reduced Graphene Oxide Composite Bifunctional Catalyst In Zinc-air Batteries

With the energy crisis and environmental pollution problems becoming increasingly serious,research and development of green,efficient,and clean high-performance new energy storage and conversion systems have received increasing attention.At present,in different energy storage and conversion systems,electrochemical energy has been concerned due to its low cost,fast acquisition speed,environmental friendliness,high utilization efficiency,and strong safety performance.Electrochemical energy systems with high specific energy density and high specific power,represented by zinc-air batteries,are favored.However,its scale and commercialization still face many challenges,the biggest problem of which is the slow kinetic process of oxygen in the battery cathode during the oxygen reduction and oxygen evolution reaction,resulting in low energy efficiency.Therefore,the development of efficient electrochemical reactions Bifunctional catalysts are particularly important.In this paper,improved Hummers method is used to prepare graphene oxide,and nickel-cobalt oxide/reduced graphene oxide?NiCo₂O₄/RGO?,nickel-cobalt sulfide/reduced graphene oxide?NiCo₂S₄/RGO?and cobalt-manganese oxide/nitrogen-doped reduced graphene oxide?CoMn₂O₄/N-RGO?electrocatalyst.The effect of the quality of graphene oxide on the morphology and electrocatalytic activity of the prepared catalyst was further studied.X-ray diffraction?XRD?,Raman,SEM,TEM,physical absorption and desorption specific surface area measurement?BET?,X-ray photoelectron spectroscopy?XPS?and other method were used to characterize the micromorphology,composition,structure,and specific surface area of the as-prepared catalyst;meanwhile,linear scanning voltammetry curves?LSV?The catalyst's oxygen reduction?ORR?and Oxygen evolution?OER?electrocatalytic activities were studied.In view of the feasibility of the prepared composites to have dual function electrocatalysis for ORR/OER,and a long-term constant current charge and discharge method,the electrocatalytic performance of the catalyst for zinc-air batteries was evaluated.The main research contents and results of this paper are summarized as follows:?1?NiCo₂O₄/RGO_0.075 was synthesized by one-step hydrothermal reaction and high-temperature calcination.The catalyst exhibited acacia flower-like nanoneedle structure,with a large specific surface area(83.15 m² g^-1)and a relatively uniform pore size distribution.The ORR and OER electrocatalytic activities of the catalyst were studied using LSV curves.It was found that the catalyst had a half-wave potential of 0.81 V,a ?E of 0.94 V,and a limit diffusion current density of 6.73 mA cm^-2 in a 6 mol L-1 KOH electrolyte.Tafel slope is 67 mV dec-1.Applied to rechargeable zinc-air batteries,the power density of NiCo₂O₄/RGO_0.075 is 230 mW cm^-2.At a current density of 5 mA cm^-2,the NiCo₂O₄/RGO_0.075 catalyst shows good performance in battery testing.cyclic stability.Compared with the commercial Pt/C+IrO₂ cathode,the NiCo₂O₄/RGO_0.075 cathode has high charge and discharge efficiency,and the minimum voltage gap?AV?value is only 0.73 V.At the same time,NiCo₂O₄/RGO_0.075 has good charge-discharge cycle stability in 35 hours.The results show that during the ORR/OER reaction,the large surface area and mesoporous structure of NiCo₂O₄/RGO provide more catalytic active sites and transport channels for reactants and electrolytes,thereby improving the stability and electrocatalysis of the catalyst active.?2?NiCo₂S₄/RGO_0.02 was synthesized through two-step hydrothermal reaction and high-temperature calcination.The composite catalyst showed a thin layered structure and a layered structure with a large surface area.Its specific surface area(92.45 m² g^-1)is improved compared to NiCo₂O₄/RGO_0.075.Compared with a single NiCo₂O₄,the sulfurization process changes the electronic state of the metal oxide and generates more oxygen defects,thereby effectively improved the ORR/OER activity.The strong synergy between NiCo₂S₄ and RGO further promoted the oxygen reaction process.The results showed that the NiCo₂S₄/RGO_0.02 bifunctional catalyst showed excellent ORR/OER activity??E:0.91 V?and stability,and assembled it in a zinc-air battery,the measured power density is 262.6 mW cm^-2,and NiCo₂S₄/RGO_0.02 has a long-term charge-discharge cycle stability at 50 h at a current density of 5 mA cm^-2.The study revealed that the ultra-thin layer structure of the composite of NiCo₂S₄ nanoparticles and graphene nanosheets greatly enhanced the surface exposure of the active material,thereby improving the charge transport path.?3?Using melamine as the nitrogen source,a nitrogen-doped CoMn₂O₄/N-RGO_0.05 dual-function electrocatalyst was synthesized by one-step hydrothermal method and high-temperature calcination.The CoMn₂O₄ metal oxide has a spherical structure,and spherical CoMn₂O₄ nanoparticles are supported on nitrogen-doped graphene nanosheets to form a thin layer of graphene-encapsulated spherical structure.The specific surface area of the catalyst(95.60 m² g^-1)is further improved compared to NiCo₂S₄/RGO_0.02.At the same time,the carbonization caused by high-temperature calcination also has a good electrical conductivity.Electrochemical results show that the CoMn₂O₄/N-RGO_0.05 electrocatalyst has higher ORR and OER performance??E:0.78 V?.When the zinc-air battery voltage gap is increased from 0.52 V to 0.70 V,the power density is as high as 354 mW cm^-2,which is more than twice the value of the precious metal Pt/C+IrO₂.When the charge-discharge current density is 5 mA cm^-2,CoMn₂O₄/N-RGO_0.05 is within 200 hours,indicating that it has good cycle stability.Further research found that the main catalytic activity of the catalyst samples was various nitrogen-carbon structures such as pyridine nitrogen,pyrrole nitrogen,and graphite nitrogen in nitrogen-doped graphene.