Preparation Of Chitosan-derived Carbon Carbon Composites And Their Electrocata-Lytic Performance For Oxygen Reduction

Oxygen reduction(ORR)is a fundamental and important electrochemical reaction process involved in the field of new energy.The development of low-cost and high-activity ORR electrocatalysis is of great significance for realizing practical applications of fuel cells,zinc-air batteries and the like.In recent years,many nitrogen-containing organic-derived catalysts have been considered as a substitute for precious metal catalysts because of their high catalytic activity,wide source,low cost,and mass production.Among them,chitosan has a large amount of amino groups and hydroxyl groups in the molecular chain,and its yield is second only to cellulose.Applying it to the field of electrocatalysis,it is expected to produce a highly active,low cost ORR catalyst.In this paper,carbon black(Vulcan XC-72),graphitized carbon black(GCB),carbon black(BP2000)is used as the base carbon material,and the chitosan-derived porous carbon is coated by high-temperature pyrolytic carbonization and activation.On the surface of the base material,a series of carbon-carbon composite materials with a core-shell structure were prepared.When the mass ratio of the three carbon substrates to chitosan was controlled to be 1:20,the GCB-based material showed the best activity,and the half-wave potential was 0.91 V vs RHE.Furthermore,a high-graphitized natural spherical graphite was used as a raw material,and a carbon-carbon composite material coated with chitosan-derived carbon was prepared by hydrothermal carbonization and activation.When the mass ratio of natural graphite to chitosan is 1/20,the activity is optimal,and the half-wave potential is 0.93 V vs RHE.The catalyst delivers a maximum power density of 167 mW cm-2 in a zinc-air battery,which is higher than commercial Pt/C(159 mW cm-2)and has excellent rate performance.The well-developed pore structure of chitosan-derived carbon,high-density active sites such as pyridinium nitrogen and graphitride nitrogen,and high degree of graphitization of carbon matrix are the key factors affecting the electrocatalytic activity of materials.In addition,special core-shells are also available.It is beneficial to increase the catalytic activity of oxygen reduction.Then,Mn,Fe,Co transition metal(TM)heteroatoms were introduced on the surface of the composite,and TM-Nx active unit structure was formed on the surface of the core-shell catalyst.Iron doping is most beneficial to improve the catalytic performance of the material.The Fe-NGCS has a half-wave potential of 0.96 V,which is higher than Pt/C 80 mV,and undergoes an ideal 4-electron path in the ORR process.It also has durability and methanol poisoning resistance superior to Pt/C.This is related to the formation of high density,highly dispersed Fe-Nx active unit structures.Finally,the effect of iron-based compounds on the catalytic performance of ORR was studied.Carbon-based iron-based compound catalysts were prepared by thermal decomposition and thermal reduction using carbon black(Vulcan XC-72),ferric nitrate and o-phenylenediamine as raw materials.Compounds with Fe3C/Fe2O3/Fe three-phase heterostructures can improve ORR catalytic activity more than Fe3C and Fe2O3.The O-Fe-C-Fe-C bond,which is attributed to the chemical interaction between the heterostructure and the matrix,acts as an active site and promotes electron transfer.