The Preparation Of Transition Metal-nitrogen Doped Carbon Materials And Their Electrocatalytic Performance For Oxygen Reduction Reaction

Oxygen reduction reaction（ORR）is an essential reaction in electrochemistry because of its importance in the cathodic reaction in polymer electrolyte membrane fuel cells（PEMFCs）.At present,Pt-based catalysts have been regarded as the best catalyst for ORR.However,the poor durability,high cost and easy poisoning of Pt catalysts hinder the commercial application of PEMFCs.To address these issues,one promising strategy is to design and synthesize low cost,high performance and good durability non-noble metal cathodic catalyst.Herein,a series of transition metal-nitrogen doped carbon materials have been designed and prepared using cheap raw materials and simple methods.The morphology,structure,composition and ORR performance of these materials were studied.The main contents and conclusions of this thesis are as follows.（1）Uniform cobalt and nitrogen co-doped carbon nanospheres with high specific surface area have been prepared by a simple but efficient method.The experimental results showed that the dosage of TETA is vital for the formation of nanospheres,and the amount of cobalt chloride has an impact on the morphology and properties of carbon nanospheres.By analyzing and comparing,the preparation conditions of catalyst have been optimized,the as-prepared catalyst（0.14CoN-CNS）showed the uniform size,high specific surface areas（865 m2 g-1）,and the best ORR activity in both alkaline and acidic electrolytes.In alkaline electrolyte,the as-prepared catalyat has more positive half-wave potential and larger kinetic current density than commercial Pt/C.In acidic electrolyte,this catalyst also showed good ORR activity with high electron transfer number,its onset and half-wave potentials are all close to those of commercial Pt/C.The as-prepared catalyst showed more superior stability and higher methanol-tolerance than commercial Pt/C both in alkaline and in acidic electrolytes.The potassium thiocyanate-poisoning test further confirmed that both nitrogen doped carbons with high specific surface areas and cobalt-nitrogen sites are active centers in alkaline electrolyte,while the cobalt-nitrogen sites are dominating to endow high ORR catalytic activity in acidic electrolyte.（2）Iron and nitrogen co-doped carbon nanosheets supported iron-based nanoparticles（Fe/Fe3C）have been obtained via pyrolysis of chitosan,urea and ferric chloride.The experimental results showed that the dosage of ferric chloride and the pyrolysis temperature have important influence on the ORR activity of these materials.By analyzing and comparing,the preparation conditions of catalyst have been optimized.The optimized material（Fe0.5NC-800）has a large specific surface area（254.7 m2 g-1）,and many 4 nm Fe-base nanoparticles covered on its surface.In alkaline conditions,the Fe0.5NC-800 showed excellent ORR activity and selectivity,its peak potential,onset potential and half-wave potential are similar to those of the commercial Pt catalyst,and the peak current density and apparent current density are slightly better than those of the commercial Pt/C catalyst,the H₂O₂ yield is lower than 7%.At-0.60 ^-0.40 V potentials,its kinetic current density（36.03 52.85 mA cm-2）are greater than that of the commercial Pt/C catalyst（18.67 20.23 mA cm-2）.After 5000 CV cycles,the ORR activity of this catalyst almost remains unchanged.The acid-washing experiment proved that these Fe/Fe3 C nanoparticles play a significant role in the ORR activity of this catalyst.