Preparation Of Nitrogen-doped Graphene Catalysts And Their Electrocatalytic Performance For Oxygen Reduction Reaction

Fuel cell has attracted wide attention for its high specific energy,high power generation efficiency,wide fuel range and environmental friendliness,but its high cost and low electrode reaction rate limit its large-scale application.The low reaction rate of fuel cell electrode is mainly due to the slow kinetic rate and low energy conversion efficiency of cathode oxygen reduction reaction（ORR）,so efficient and stable electrocatalysts are needed to solve this problem.At present,noble metal platinum catalyst is the most commonly used catalyst and the only one to realize commercialization,which has significant ORR catalytic activity and can effectively reduce ORR activation energy.However,due to the low reserves and lack of resources,The Pt-based catalysts are still costly,and the rapid attenuation of electrocatalysis process and CO poisoning effect of Pt-based catalysts indirectly increase the cost and hinder its wide application.Therefore,i t is of great significance to develop a metal-free catalyst with low cost,high activity and high stability.Nitrogen-doped graphene material is the most widely used metal-free catalyst.On the one hand,nitrogen doping itself can form a variety of catalyt ically active nitrogen species,and the size of nitrogen atoms is similar to that of carbon atoms,which can change its electronic structure on the premise of minimizing lattice mismatch.On the other hand,compared with other carbon-based materials,graphene is expected to become a better substrate due to its thin lamellar structure,high specific surface area,good electrical conductivity and high charge mobility,which can promote the full exposure of active sites,enhance the electron transmission,redu ce the reaction barrier and then achieve the purpose of enhancing ORR catalytic activity.In this paper,graphitic carbon nitride（g-C₃N₄）and graphene were used as raw materials to prepare nitrogen-doped graphene materials by in-situ synthesis and direct doping respectively,and the effects of different factors on the ORR catalytic performance were discussed.The main contents are as follows:（1）Using graphitic carbon nitride（g-C₃N₄）as raw material,magnesium metal（Mg）powder as reducing agent,g-C₃N₄ with semiconductor characteristics was converted into nitrogen-doped graphene（N-G）material with stronger conductivity and higher electron transmission efficiency through the reaction of Mg and N atoms in g-C₃N₄ at high temperature,which was then used a s ORR catalyst.TEM,XRD,XPS,FI-IR and BET specific surface area were used to characterize the morphology,structure,chemical composition and pore structure of the materials.ORR catalytic activity and stability of different catalysts were studied by CV（cyclic voltammetry curve）,LSV（linear voltammetry sweep curve）,K-L（koutech-levich equation）curve and i-T（chronoamperometry）curve,and the practical application value of the catalyst was analyzed by Zn-air battery test which was then compared with c ommercial 20 wt.%Pt/C.The results showed that compared with g-C₃N₄,the sheet of N-G was significantly thinner and the specific surface area increased by nearly 30 times after high temperature magnesium reduction.The obtained N-G had good ORR catalytic activity,stability and methanol resistance.In addition,the performance of Zn-air battery composed of N-G catalyst is closed to that of commercial 20 wt.%Pt/C,indicating that the N-G catalyst has practical application value.（2）Using graphitic carbon nitride（g-C₃N₄）as the nitrogen source,graphene as the carbon matrix,the stable composite precursors were formed byπ-πconjugation and hydrogen bonding of g-C₃N₄ and GO structures in hydrothermal process,different kinds of nitrogen-doped graphene（N-Gr）catalysts were obtained after high-temperature heat treatment.The morphology,structure,chemical composition and pore structure of the materials were characterized by SEM,XRD,XPS and BET specific surface area.The ORR catalytic activity,stability a nd practical application value of catalysts were studied by CV,LSV,K-L curve,i-T curve and Zn-air battery test.The results show that N-Gr catalysts prepared by hydrothermal and high-temperature heat treatment can be doped with more nitrogen substances and has good ORR catalytic activity and stability in alkaline electrolyte.When assembled into Zn-air battery,the N-Gr catalyst shows close power density and dynamic response than commercial 20 wt.%Pt/C,indicating its practical value.