Preparation And Electrocatalytic Performance Of Novel Nitrogen-doped Graphene-based Nanocomposites

Due to the massive use of fossil fuels,the energy crisis and environmental pollution have become increasingly serious.In order to meet the ever-increasing energy demand and solve the resulted environmental problems,many researchers are committed to green and efficient clean energy technologies-fuel cells,zinc-air batteries and electrocatalytic water splitting systems.Among them,electrocatalytic oxygen reduction（ORR）,oxygen evolution（OER）and hydrogen evolution reaction（HER）are essential for the development of new energy technologies.However,these reaction kinetics are slow,resulting in low conversion efficiency.For this reason,a catalyst that can accelerate the reaction rate and highly needed.As we all know,Pt is recognized as the most effective commercial catalysts for ORR and HER.Ir O₂and Ru O₂are the most effective commercial catalysts for OER.Due to the scarcity,high price and poor durability of these precious metals,their industrial applications are severely restricted.Therefore,the design and development of low-content precious metal catalysts with high activity,high stability,and low cost is of great significance to future sustainable development.In order to prepare high-efficiency catalysts,the usual strategy is to adjust the morphology,size,composition,crystal form,etc.of the material to effectively adjust the electronic structure and surface properties of the catalyst,thereby improving its catalytic performance.In addition,the carbon substrate has high electrical conductivity,acid and alkali resistance,and is easily compounded with metals,which has potential application prospects in the field of electrocatalysis research.Based on this,this thesis mainly composites transition metals and transition metal compounds with nitrogen-doped graphene carbon materials,and studies their electrocatalytic performance and catalytic mechanism.The main research contents are as follows:（1）Nitrogen-doped graphene with abundant nitrogen sites was used as a catalyst carrier.At room temperature,Ru nanoclusters（amorphous,bipolar amorphous/crystalline,crystalline）with different crystallicity were prepared by ion adsorption and heat treatment strategies.Nanohybrid materials were anchored on nitrogen-doped graphene.Its structure,morphology and microstructure are characterized by XRD,HRTEM,XPS,Raman etc.The prepared amorphous/crystalline Janus Ru nanoclusters have excellent HER(10 m A·cm^-2corresponding to an overpotential of 20.1 m V)and ORR(E_1/2=0.847 V)bifunctional catalytic performance,as well as excellent the stability(HER:after 2000 CV cycles,overpotential is increased by 2 m V at 10 m A·cm^-2,ORR:after 10000 CV cycles,the half-wave potential is reduced by 17 m V).Compared with commercial Pt/C and reported Ru based electrocatalysts,the amorphous/crystalline Janus Ru nanoclusters/N-G have excellent electrocatalytic activity and durability.The constructed amorphous/crystalline Janus-like Ru structure not only causes a large number of surface active sites to be exposed,but also accelerates the electron transfer in the electrocatalytic process.Density functional theory（DFT）calculations show that N-coordinated amorphous/crystalline Janus Ru nanoclusters can promote the formation of OOH*and H₂O*,thus its ORR and HER have superior catalytic activities.This work provides new ideas for the design and manufacture of various new metallic amorphous/crystalline Janus nanostructured materials for energy conversion applications.（2）At room temperature,Fe³⁺was added to the ultrasonically dispersed graphene solution,and Fe@nitrogen-doped graphene composite nanomaterials were prepared by ion adsorptionan and subsequent carbonization treatment.Use XRD,SEM,HRTEM,XPS,Raman and other techniques to characterize its morphology,structure and surface characteristics.The experimental results found that high-temperature carbonization formed Fe nanoparticles,and low-temperature carbonization formed Fe single atoms.Due to the higher atomic utilization efficiency of Fe single atom,the single atom Fe@nitrogen-doped graphene composite nanomaterial showed better ORR catalytic performance in 1 M KOH solution,with a half-wave potential(E_1/2)of 0.88 V and a limiting current density of 6 m A·cm^-2,which was even better than commercial Pt/C.After10,000 cycles,the half-wave potential only has a negative shift of 13 m V,and after the10,000 CV cycle test,its current density hardly decays.However,the commercial Pt/C has passed 2000 CV,and the half-wave potential has been 18 m V.The negative shift,the current density decreased by 0.5 m A·cm^-2,indicating its long-lasting catalytic performance and structural stability.Its excellent electrocatalytic performance is attributed to the coordination of nitrogen and Fe in defective nitrogen-doped graphene to form strong Fe-N structure,which prevents particle aggregation during the electrocatalytic process.In addition,the single atom Fe component can contribute more catalytic activity,which can improve the catalytic activity and stability of the material.The synthesis method of this research work is simple and environmentally friendly,which provides a new attempt for the design of other high-performance single-atom catalysts.（3）Inspired by the optimization of electronic structure to improve the performance of the catalyst,the transition metal phosphide Co P was selected as the research object and doped with Fe to prepare Fe-doped Co P nanocomposites on nitrogen-doped graphene.And different dopings were explored the effect of the amount on the catalytic performance of Co P/NG.We carried out XRD,SEM,TEM,XPS and other tests on the product,which proved that the incorporation of Fe into it caused a change in the electronic structure of Co P.The electrochemical performance of the catalyst was tested,and the results showed that when the added Co:Fe molar ratio was 0.28:1,the Fe-doped Co P/NG exhibited the best HER and OER performance in a 1 M KOH solution.Compared with Co P/NG(the overpotentials of HER and OER are 207 m V and 381 m V at current density of 10 m A·cm^-2,respectively),the HER and OER performance of Fe-doped Co P/NG is significantly improved(10 m A·cm^-2current density,the overpotential of HER/OER is 173 m V and 310m V respectively).This work uses heteroatom doping to change the electronic structure of the catalyst,optimizes the binding ability of water molecules and reaction intermediates,thereby improving the catalytic performance,and provides a new idea for improving the catalytic performance of transition metal compounds.