Reparation And Electrocatalytic Performance Of Cobalt-based Nitrogen-doped Mesoporous Carbon Materials

Fuel cell system has been regarded as the most promising hydrogen energy conversion system because of high efficiency of energy conversion,friendly environmental,quiteness,etc.To date,Pt-based catalysts have remained the best performance electrocatalyst towards oxygen reduction reaction(ORR).Nonetheless,the fatal disadvantages like scarcity,high cost,and liable to poisoning,are serisously hindering the commercialized development of fuel cells.It is of important and increasing interest to develop alternative non-noble metals electrocatalysts and then promote the commercial applications of fuel cells.Among numerous non-noble metals electrocatalysts towards ORR,transition metal-nitrogen-carbon(M-N-C)compounds have recently attracted extensive attention of domestic and overseas scientific researchers because of low price,comparable catalytic activity and stability to that of Pt-based catalysts,and high tolerance towards small organic molecules.This kind of catalyst is usually prepared by pyrolysis of nitrogen-containing precursor under certain atmosphere,and its catalytic performance including activity and stability is depending on chemical composition of precursor,pyrolysis temperature and atmosphere,etc.However,it remains doubts on the catalytic mechanism of electrocatalysis.Hence,it is of great theoretical significance and practical application value for commercialized development of fuel cell to explore structure design and controllable preparation methods of novel high-performance non-precious metal electrocatalysts.In this work,two kinds of N-doped carbon-supported Co-based electrocatalysts were prepared under optimazied pyrolysis temperature and metal mass loading by one-step pyrolysis using glucose,colbat salt and ethylenediaminetetraacetic acid(EDTA)or melamine as carbon source,metal source and nitrogen source,respectively.To gain insight in influence of factors such as carbon support,nitrogen source,metal loading and pyrolysis temperature on electrocatalytic performance,multiple characterization techniques were carried out to analyze chemical composition,structure,morphology,and ORR performance of the prepared catalysts.The purpose of this work was to reveal the structure-performance relationship and the synergistic effects between active components,which would provide scientific basis for the design of chemical composition and structure for novel high-performance N-doped carbon-supported Co-based electrocatalysts.The main contents were as follows:(1)A series of bifunctional N-doped Co-based electrocatalyst C-T(T ?500,600,700,800 and 900 °C,pyrolysis temperature)were prepared using Co-EDTA/glucose as carbon and nitrogen sources,colbat nitrate as cobalt source in a one-step pyrolysis route.The influence of pyrolysis temperature and nitrogen specises on electrocatalytic performance toward ORR and OER was mainly examined.The results exhibited that:the prepared catalyst C-600 exhibited uniform distribution of Co and N elements,the highest nitrogen content(9.79 at%),large specific surface area(423 m2 g-1)and the most defects(ID/IG ? 1.93);synergistic effects between N-doped carbon and Co/CoOx boosted the ORR activity;among the five C-T samples,C-600 showed the best ORR and OER bifunctional activity and stability in KOH electrolyte,for example,its onset potential,half-wave potential and diffusion-limited current at 1600 rpm were 0.87 V,0.76 V,5.6 mA cm-2,repectively,comparable to that of commercial Pt/C catalyst;its OER overpotential at 10 mA cm-2 was 390 mV,comparable to that of commercial IrO2/C catalyst.(2)A series of N-doped carbon nanotubes/carbon spheres supported Co-based hybrid CNCo-x(x = 10mco/mcss,mass ratio)were prepared by the calcination of mixture of Co-based carbon spheres precursors and melamine,w hich were obtained by coprecipitation method with nanocarbon spheres from glucose as carbon support,cobalt acetate as metal source and sodium dodecyl sulfonate(SDS)as morphology stabilizer.The effects of different metal loading and acid etching for CNCo-x samples on ORR performance were investigated.The results exhibited that:CNCo-3 displayed the highest nitrogen content(10.03 at%),large specific surface area(550 m2 g-1)and the most defects(ID/IG = 1.85);trace Co species greatly enhanced the ORR activity of materials and the optimized value of metal loading mco/mcss was 0.3;CNCo-3 demonstrated high ORR activity,for example,its onset potential,half-wave potential and diffusion-limited current at 1600 rpm are 0.93 V,0.83 V,4.8 mA·cm-2,repectively,comparable to that of commercial Pt/C catalyst and better durability than Pt/C catalyst;both the onset potential and half-wave potential of acid etched CNCo-3 reduced a little but promoted a 4e-pathway towards ORR.