Preparation And Electrocatalytic Performance Of Carbon-based Non-noble-metal Catalysts

With the advent of global energy crisis,the establishment of a clean,cheap,and renewable energy system has become a focus for researchers.The studies of fuel cells,supercapacitors,metal-air batteries,and electrolytic cells play an important role in the storage and conversion of clean energy.In fact,the main factor restricting the development of electrocatalysts is the expensive cost of platinum and other precious metals as catalysts in various electrode reactions such as oxygen reduction reaction（ORR）in metal-air batteries and hydrogen evolution reaction（HER）in electrolytic cells.Therefore,the development of highly active and cost-effective non-precious-metal catalysts is the key for improving the electrocatalytic performance.As potential electrocatalysts,transition metals are commonly investigated in the past years due to their relatively high catalytic activity,low cost,and good stability.However,the weak conductivity and poor dispersibility of transition metals and their compounds have greatly hindered the development and application.The combination of transition metals（or their compounds）with highly conductive carbonaceous materials can expose more active sites,improve the conductivity and stability of the catalysts,which is an effective strategy to solve above problems.The present dissertation mainly focuses on the fabrication and electrocatalytic performance of transition metals（Co,Mo）and their compounds with carbonaceous materials.The main research contents and results of this work are as follows:1.Preparation and electrocatalytic hydrogen evolution performance of NCS@MoS₂ nanosheets.The development of pure MoS₂ nanosheets have been restricted by the shortcomings such as agglomeration and poor conductivity,which greatly inhibit their application in hydrogen evolution reaction.In this section,the co-precipitation method is employed to prepare two-dimensional ZIF-8,which is calcined at high temperature,afording the formation of nitrogen-doped porous carbon（NCS）.Then,the NCS matrix is modified and the NCS@MoS₂ composite can be assembled in the presence of MoS₂ precursors by hydrothermal method.The results show that the MoS₂ nanosheets are uniformly and densely loaded on the surface of NCS.In addition,the loading effect of outer layer of MoS₂ can be effectively controlled by changing the modification condition of NCS.The NCS@MoS₂-2 hybrid with an appropriate MoS₂ loading has exhibited the smallest HER overpotential(250 m V,10 m A cm^-2)and Tafel slope(53 m V dec^-1)as well as high stability and durability.The excellent electrocatalytic HER performance of the composite catalyst can be ascribed to the unique sheet structure and the synergitic interaction between the components.2.Preparation and electrocatalytic oxygen reduction performance of core-shell-structure HCS@Co/NCThe nanosphere template with a silicon dioxide core covered by a phenolic resin shell is synthesized according to the classic Stober method.The silicon dioxide is etched to form porous hollow carbon spheres（HCS）after a carbonizing process.Co/NC particles with high ORR activity are loaded on the surface of HCS by a hydrothermal method,and the HCS@Co/NC composite with a hetero-interface structure can be rationally constructed.The unique core-shell structure of the composite effectively alleviates the accumulation of active sites,shortens the diffusion distance of electrons in electrochemical reactions,ensures the full contact between reactive substances and electrolyte,and provides an efficient path for the transportation of electrons,oxygen and ions.The as-prepared HCS@Co/NC exhibits an excellent catalytic activity,stability and methanol resistance under alkaline condition.The half-wave potential of 0.84 V is higher than that of the commercial Pt/C catalyst.In addition,the catalyst can still maintain 90.7%of the initial current after 8 hours of electrocatalytic cycles under-0.5 V vs.RHE.3.Preparation,electrocatalytic oxygen reduction and Zn-air batteries performances of NCS@Co/CoO_x nanosheetsThe hydrothermal method is utilized to load ZIF-67 particles on the modified nitrogen-doped carbon nanosheets（NCS）.The ORR electrocatalyst composed of cobalt,cobalt oxides,and carbon nanosheets（NCS@Co/CoO_x）is constructed after a calcining treatment under N₂ gas.In the structure,the nanosheets afford a large specific surface area and matrix effect,together with the synergistic effect of multi-active Co-N_x,CoO,and Co₃O₄ sites inhibit the aggregation of active sites and enhance the conductivity of the catalyst.Above advantages endow the NCS@Co/CoOx catalyst an excellent ORR activity under alkaline condition.The half-wave potential of 0.85 V is higher than precious-metal Pt/C（0.83 V）.When the NCS@Co/CoO_x composite is employed as the air-electrode catalyst of zinc-air battery,the battery shows a better performance than that of commercial Pt/C.The maximum power density of the battery is 182 m W cm^-2,and exhibits a high specific capacity of 814.3m Ah g^-1 and energy density of 950.3 Wh kg^-1 at a current density of 10 m A cm^-2.In addition,the flexible all-solid-state Zn-air battery with NCS@Co/CoO_x as air-electrode catalyst also shows a superior stability,where the battery performance remains almost unchanged even under the bending state of 90°.