Synthesis Of Heterostructural Co-based Catalyst And Their Multifunctional Electrocatalytic Properties

Owing to global warming and energy shortages,there is an urgent need for sustainable energy conversion and storage equipment.Metal-air cells,water splitting and fuel cells have been attracted much attention due to their high energy density and energy conversion safety and environmental friendliness.The main electrode reaction in these equipments are oxygen reduction（ORR）,oxygen evolution reaction（OER）,and hydrogen evolution reaction（HER）,which have complex multi-step electron-transfer reactions.Thus,they have slow dynamics and low reaction efficiency,which limit the operating efficiency of the equipments.Loading an efficient catalyst is one of the effective methods to enhance operating efficiency of the battery system.Although noble metal-based catalysts such as Pt-based catalysts and Ru-based catalysts have been widely used in ORR,HER and OER,noble metal-based catalysts have the disadvantages of scarce resources,high cost and poor stability,thus hindering their large-scale application in energy conversion devices.Therefore,it is of scientific significance and practical prospect to explore non-noble metal based catalysts with high catalytic activity and economic benefits.In this paper,various non-noble metal heterostructure catalysts were designed and synthesized by direct pyrolysis.The electrocatalytic properties of non-noble metal heterostructural catalysts toward ORR,OER and HER are investigated.And the practical performance of zinc-air batteries based on heterostructural catalysts is also studied.The main research results of this paper are as follows:1.By using titanium dioxide,dicyandiamide（DCD）,cetyltrimethyl ammonium bromide（CTAB）,cobalt salt as raw materials,a heterostructure between Co and Ti N was formed and deposited on N-doped carbon carrier to obtain the Co||Ti N/NC composite by high-temperature pyrolysis.The Co||Ti N/NC composite has an excellent electrocatalytic activity toward ORR with half-wave potential E_1/2=0.81V,current density J=5.69 m A cm^-2,which is comparable to commercial Pt/C(E_1/2=0.81V,J=5.57 m A cm^-2).The excellent catalytic performance of the Co||Ti N/NC composite is attributed to the heterostructure between Co and Ti N.It can optimize the electronic structure to improve the electron transfer efficiency between different components and create more active sites to improve the electrocatalytic performance.Moreover,the N-doped carbon carrier synthesized in situ further enhances the conductivity and stability of catalyst in catalytic process.2.Through high-temperature pyrolysis of Co,W precursor and DCD,Co@WC_1-xheterogeneous nanoparticles were attached to self-generated N-doped carbon nanotube to form Co@WC_1-x/NCNTs composite.Self-generated N-doped carbon nanotubes（NCNTs）can provide excellent electron conduction capability for electrocatalytic process.The active metal components loaded on the walls of NCNTs to form a rapidly moving electronic interface(NCNTs→WC_1-x→highly active site Co),which helps to enhance the electrocatalytic activity of catalysts.Therefore,Co@WC_1-x/NCNTs composite showed excellent electrocatalytic activity for ORR with E_1/2=0.81V,J=5.60 m A cm^-2.It also exhibited excellent electrocatalytic activity and stability toward OER with overpotentialη₁₀=330 m V.The test on self-assembled zinc-air battery showed that the zinc-air battery had a high energy density,specific capacity,and excellent charge-discharge stability when using Co@WC_1-x/NCNTs composite as an electrocatalyst.3.By using cobaltous oxide,DCD,CTAB,copper salt as raw materials,Co||Cu/NC composite was successfully prepared by high-temperature pyrolysis,in which heterogeneous particles Co||Cu coated with N-doped carbon.The Co||Cu/NC composite has excellent electrocatalytic activity for ORR with E_1/2=0.83V,J=5.79 m A cm^-2and for HER withη_-10=210m V.The excellent catalytic properties of Co||Cu/NC composite are attributed to the following possible reasons:（1）Cu was introduced as a second metal to form Co||Cu heterogeneous particles,which can accelerate the charge transfer between Cu and Co to enhance the catalytic reaction process.（2）Nitrogen-doped carbon layer can prevent the loss of active components to improve the catalytic stability.（3）The synergistic coupling action between the metal and the carbon layer can optimize the electronic structure of the catalytic surface to improve electronic transfer capability and enhance the catalytic activity.4.By using Mo S₂nanosheet（via ultrasonic stripping）,cobalt salt,phosphorus source,and DCD as raw materials,Mo S₂||Co P heterostructure was formed and deposited on N,P co-doped carbon（NPC）to obtain Mo S₂|Co P/NPC composite under high-temperature pyrolysis.Because of the synergistic effect between Mo S₂|Co P heterostructure and N,P co-doped carbon,the Mo S₂||Co P/NPC composite shows excellent multifunctional electrocatalytic activity toward ORR(E_1/2=0.79V,J=5.47 m A cm^-2),OER(η₁₀=306 m V)and HER(η_-10=143 m V).It provides very promising possibility of practical application for the hydrogen production of water splitting and zinc-air battery.5.The Co||Mn Co₂O_4.5/NC composite was prepared by high-temperature pyrolysis of spinel Mn Co₂O_4.5and DCD.Electrochemical experimental results show that the Co||Mn Co₂O_4.5/NC composite has an ultra-high reaction current(J=6.14 m A cm^-2)and a half-wave potential（0.81 V）for ORR.And it also has a low overpotential(η₁₀=330 m V)for OER.The superior electrocatalytic acticity of the Co||Mn Co₂O_4.5/NC composite may be ascribed to following points.Firstly,the heterostructural Co||Mn Co₂O_4.5will provide more active adsorption sites and accelerate electrons to transfer rapidly in the interface.Secondly,the appropriate oxygen vacancies can cause better catalytic performance because of their positive effects,such as changing electronic structure,optimizing electrical conductivity and regulating the active sites.Thirdly,rich N-doped carbon can be used as a conductive carrier to accelerate electron transfer.In conclusion,non-noble metal Co（or Co P）was used as the main component,combining with the second transition metal,transition metal carbides,nitride,sulfide or spinel oxide to construct heterostructural composite,and using hetero-atom doped carbon material as a carrier to obtain five heterogeneous non-noble metal composite electrocatalysts.Five non-noble metal heterogeneous electrocatalysts have excellent multifunctional electrocatalytic activity for ORR,OER or HER.Furthermore,they present high energy density,discharge capacity and outstanding charge-discharge stability for application of zinc-air batteries.The research results of this paper clarify the effect of heterogeneous structure for improving the catalytic performance of catalysts.It provides significative scientific theory and experimental basis on the development of non-noble metal multifunctional catalysts with high efficiency and economy.