Controllable Preparation Of Transition Metal/carbon Based Composite Materials And Research On Their Electrocatalytic Performance

The excessive consumption of fossil energy and the emission of carbon dioxide have caused serious energy crisis and environmental problems,which make it more and more urgent for people to develop new energy.Fuel cells have been regarded for a long time as ideal energy devices with high efficiency and low pollution,which can provide sufficient mileage for electric vehicles.Zinc-air,lithium-carbon dioxide and other batteries have also attracted extensive attention of researchers due to the open energy source and high energy density.However,the operating efficiency of these batteries and fuel cells is often limited by the slow kinetic process of electrode reactions,thus requiring a large amount of precious metals to improve the reaction rate,which seriously hinders the process of large-scale commercialization.Therefore,the development of inexpensive non-noble metal catalysts to improve the electrode reaction efficiency of fuel cells（oxygen reduction）,zinc-air batteries（oxygen reduction/evolution）and lithium-carbon dioxide batteries（carbon dioxide reduction/evolution）has become a research hotspot.Carbon-based materials are the popular materials for catalytic reactions due to their high stability,low price and simplicity to structure regulation.And the introduction of metal sites will further enhance the activity of carbon-based materials.Based on this fact,this thesis designed a series of structure-oriented controllable preparing strategies,and synthesized several non-noble metal/carbon composites with various functions to provide new ideas for the development of the above energy devices:1.A novel method was developed for in-situ preparation of platinum-like catalyst（Tungsten carbide）under inert atmosphere and low thermal treatment temperature.With carbon source,tungsten source,iron salt and structure guide agent as precursors,tungsten-iron carbide loaded on ordered mesoporous carbon（Fe-W-C）can be obtained after solvent-evaporation-induced self-assembly（EISA）,thermal polymerization and heat treatment processes.Differing from the traditional process of preparing WC,through a carbon thermal reduction（1000 ^oC）or hydrogen reduction,this thesis could synthesize WC at a low temperature of 750 ^oC,with the in-situ catalytic effect of iron salts.Through testing the polarization curve of oxygen reduction reaction（ORR）in alkaline media,it can be found that Fe-W-C catalyst exhibits good ORR catalytic activity and the half-wave potential(E_1/2)gap of Fe-W-C and commercial 20 wt%Pt/C is less than 100 m V,indicating a great improvement compared with the pure WC material.Similarly,cobalt salt is used instead of iron salt and nitrogen source is introduced to obtain another composite material（Co-W-C/N）,consisting of nitrogen-doped ordered mesoporous carbon incorporated with WC and Co.Its ORR catalytic performance is further improved compared with Fe-W-C.And the E_1/2gap of Co-W-C/N and Pt/C is as small as 47 m V.2.A novel special Prussian blue analogue（Zn₃[Fe（CN）₆]₂）can be prepared through a simple precipitation reaction by bridging the zinc ions with the cyano ligand,thus being used as the precursor of Fe@N-C catalyst.During the process of heat treatment activation,the volatilization of Zn in high tempreture helps to enlarge the specific surface area of the catalyst and to promote the dispersion of active species metallic iron/iron carbides,thus greatly enhancing the catalytic activity of ORR.The Fe@N-C catalyst obtained after heat treatment is made up of porous nitrogen-enriched carbon as well as Fe/Fe₅C₂ wrapped inside the carbon（Fe/Fe₅C₂@N-C）.The abundant doping nitrogen and the special core-shell structure give Fe/Fe₅C₂@N-C excellent catalytic activity and stability for ORR.The polarization curves measured in basic electrolyte show that the E_1/2 of the ORR polorization curve for Fe/Fe₅C₂@N-C is as high as 0.85 V（vs.RHE）,and the number of transferred electron is close to4.These catalytic parameters are comparable to commercial 20 wt%Pt/C.In addition,after 5000potential cycles,Fe/Fe₅C₂@N-C still maintains a polarization curve similar to the initial state.Based on the high activity and stability of Fe/Fe₅C₂@N-C,we can apply it to a zinc-air battery,which delivers a high power density and specific capacity.3.A fast preparation method is designed to synthesize nitrogen-doped carbon nanotubes through the pyrolysis of a cobaltic Prussian blue analogue（Co₃[Co（CN）₆]₂）and the active sites of the resultant Co@N-CNT catalyst are also discussed.The bamboo-like carbon nanotubes are enriched with cobalt and nitrogen,wherein the cobalt nanoparticles are wrapped inside the tips of nitrogen-doped carbon nanotubes.Demonstrated by the electrochemical data acquired in alkaline electrolyte,Co@N-CNT has bi-functional catalytic ability to promote both ORR and oxygen evolution reaction（OER）.The E_1/2 value of the ORR polarization curve is 0.85 V（vs.RHE）,which is better than 20 wt%Pt/C.And the over-potential at the current density of 10 m A cm^-2 in the OER polarization curve is 479 m V,similar to that of 475 m V on Ru O₂ electrode.By means of electrochemical cleaning,cobalt particles inside carbon nanotubes can be removed.Furthermore,the importance of cobalt particles in the catalytic reactions was proved by control experiment.In combination of X-ray absorption spectroscopy test and first-principles calculation,it can be found that the catalytic center of Co@N-CNT is on the wall of carbon nanotubes,while the Co particles inside the carbon tubes also play an important role.The cobalt particles can promote ORR to some extent,but have a vital effect on OER.When Co@N-CNT is used as the catalytic material of air-cathode,the rechargeable Zn-air battery displays betther power density and cycle stability than the commercial Pt/C-Ru O₂.4.On the basis of research on oxygen catalysis,the research is extended to the field of carbon dioxide utilization.Two hybrid types of carbon naotubes are designed in this thesis to help Li-CO₂batteries achieve high capacity and long life.In a traditional coin-cell,the battery with Co,N enriched carbon nanotubes（Co-N-CNTs）as the cathode shows a better performance than commercial carbon nanotubes,delivering a capacity of up to 6300 m Ah g^-1 and stably running for 92times.In addition,a polymer gel electrolyte is introduced to propose a concept of quasi-solid lithium-carbon dioxide battery,whose capacity and life have been greatly improved,compared with that of liquid battery.Using highly active nitrogen-sulfur co-doped carbon nanotubes（N,S-doped CNTs）as the cathode catalytic material,the quasi-solid battery can release an ultra-high capacity of 23560m Ah g^-1.During the discharge process,the gel electrolyte leads to the formation of small-size lithium carbonate,which can reduce the polarization of electrode.As a result the battery can be discharged and charged for 538 times and the whole run time is more than 110 days.Due to the well-designed battery system,this quasi-solid CO₂ battery can perform as a wearable device,showing excellent mechanical flexibility.