Preparation And Performance Of Nitrogen-doped Porous Carbon-based Electrocatalysts By Mechanochemical Method

Replacing traditional fossil energy with green and clean sustainable energy has become the trend of the times.Among many sustainable energy sources,electrochemical energy equipments,such as fuel cells,metal-air batteries and supercapacitors,can convert chemical energy into electrical energy and store/release it.Because of its wide application and strong practicability,it has become the most likely substitute for fossil energy without pollutant emissions.Oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?are two important reactions for fuel cells and metal-air batteries.Noble metal-based Pt and Ir O₂/Ru O₂catalysts are the most effective ORR and OER electrode materials.Although noble metal-based catalysts perform high ORR/OER catalytic activity,there are many problems,such as CO poisoning,easy dissolution,poor stability,scarce resources and high cost.Therefore,the development of efficient and stable non-precious metal-based ORR and OER catalysts is the key to breaking the bottleneck of fuel cell and metal-air battery.In this paper,nitrogen doped porous carbon?NPCs?were synthesized by mechanochemical method and modified by structural optimization and metal doping.The mechanochemical method is low-cost,environmentally and economically friendly,which can be used to prepare highly efficient electrocatalysts.And the electrocatalytic mechanism was exploed by the analysis of its morphology,structure,composition and theoretical calculation.The main results are as follows:?1?The N-containing polymer was synthesized via ball-milling using isophthalaldehyde and p-phenylenediamine as the carbon and nitrogen sources,and Si O₂ nanospheres as the template.And at the same time,the Si O₂ templates were evenly dispersed.Subsequently,the polymer was pyrolyzed,and the Si O₂spheres were removed to obtain N-doped porous carbons with hierarchical micro-mesopores and a large surface area.The pore structure was optimized by adjusting the temperature and the amount of Si O₂ template.The results showed that the specific surface area and pore volume of NPC increased with the increase of temperature,which is mainly due to the increase of the number of micropores.When the mass ratio of Si O₂ template to carbon source is 1:1 and the heat treatment temperature is1000?,the catalyst NPC-1000 shows the most excellent ORR activity,stability and methanol resistance under alkaline conditions.When used as the air cathode of zinc-air battery,NPC-1000 perform excellent discharge performance and stability.?2?NPCs prepared by mechanochemistry combined with pyrolysis showed excellent ORR activity,but OER activity was poor.Therefore,we modified NPC by metal doping in order to achieve bifunctional ORR and OER electrocatalyst.Cobalt source or both cobalt and iron sources were added in the ball milling,and cobalt/cobalt iron and nitrogen co-doped porous carbon Co-NPC/Fe Co-NPC were obtained after pyrolysis.The results showed that although Co doping could improve OER activity,its ORR activity decreased.However,the introduction of Fe can improve both ORR and OER catalytic activity of Co-NPC.Experimental results confirmed that the ORR catalytic active sites in Fe Co-NPC were Fe-N_x,Co-N_x,pyridinic-N and graphitic-N,while Fe Co alloy particles were the main OER active sites.The results of DFT calculation show that the introduction of Fe can optimize the adsorption of oxygen-containing functional groups on Co-based catalysts.When used as cathode catalyst for rechargeable zinc-air battery,Fe Co-NPC showed better charge-discharge characteristics,energy density and cycle stability than noble metal Pt/C and RuO₂.