Study on Synthesis and Performance of Non-noble Metal Nanoelectrocatalysts

Author:Zhang Li Juan

Supervisor:wei jian feng

Database:Doctor

Degree Year:2017

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Pages:136

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The performance of cathode catalysts directly influences the efficiency of fuel cell.Pt/C has been regarded as the most efficient catalyst for oxygen reduction reaction(ORR).However,several disadvantages such as high cost,CO poisoning and poor durability largely limit its commercial application in fuel cell.Therefore,much effort still needed to explore low cost and high activity catalysts for ORR.Graphene,as two-dimensional materials with high surface area,excellent electronic,mechanical,and thermal properties,has attracted enormous attention owing to its applications in many fields:energy-storage,bio-sensing,catalysis and so on.Graphene has been found to be catalyst for ORR.Moreover,it is an ideal platform for growing or anchoring functional nanomaterials.Nevertheless,the performance of single graphene could not achieve the standards for commercial application.It has been suggested that graphene-based composite could effectively enhance the ORR activity.For this purpose,we developed several graphene-based composites as catalysts with graphene oxide(GO)as raw material.Then,we prepared graphene based metal phthalocyanine,N-doped graphene metal oxide and carbide,which could combine the performance of composites.In order to achieve better catalysts,the constitution,morphologies and electrochemical properties of composite were investigated.The main contents and conclusions are listed as follows:Using ethylene glycol as reducing agent and precursor,we prepared tetranitro-cobalt phthalocyanine/reduced graphene oxide(TNCoPc/rGO)composite by one-step solvothermal method.There is a strongπ-πinteraction between TNCoPc and rGO.By studying the catalysts derived from precursors with various GO contents and reaction times,we discovered that played important roles during the formation of morphology and structure of composite.Compared with TNCoPc and rGO,the composite had a larger peak current and a higher current density for O2 reduction in an alkaline medium,which exhibited two steps two-electron pathway for ORR.Moreover,the electrode displays good stability.A novel template-free hydrothermal approach was developed to synthesize metastableα-phase copper phthalocyanine(CuPc)nanobelts,which could be realized under a mild condition with a low reaction temperature.Results show that as-synthesizedα-CuPc nanobelts possessed well crystalline structure with long lengths above 50?m.A self-assembly formation mechanism of CuPc nanobelts were briefly discussed.Nitrogen-rich M-Nx catalyst was innovatively prepared by pyrolysis of the mixture of CuPc/GO.The optimal temperature is 750°C,there are highly active atalysts Cu-N4/rGO and Cu-N2/rGO in it.XPS anlysis indicate that contain there types of N,pyridinic N,pyrrolic N and graphitic N in Cu-Nx/rGO.We also found that the proportion of pyridinic N and graphitic N play an important roles in enhanced ORR performance.LSV results show the catalyst exhibit higher current density than Pt/C.Motivated by the mechanisms of nitrogen doped graphene,copper oxide hollow microspheres(CuO)/nitrogen-doped reduced graphene oxide(N-rGO)was synthesized by microwave hydrothermal method.It shows that porous copper oxide microspheres with particle size ranging from 1 to 3?m are uniformed grown on the surface of N-rGO.The shell of microsphere was consisted of various nanoparticles which had flower-like structure.By comparing the different precursors,we proposed a formation mechanism for the CuO hollow microspheres/N-doped rGO.The results show that pyridinic and graphitic nitrogen are exsistenced in it.The enhanced electrocatalytic activity was due to the synergy effects between N-doped rGO and hollow microspheres of CuO.A novel concept is put forward to synthesized carbon spheres/nitrogen-doped graphene/carbon nanotubes(CS/N-G/CNT).The hybrid CS/N-G/CNT displays larger BET surface 132.28 m2·g-1,pore volume 0.48 cm3·g-1,and higher diffusion limiting current density:5.51 mA·cm-2,giving an electron transfer number of 3.5(a combination of two-electron and four-electron pathways,and the four-electron pathway is the dominant one).Moreover,it shows better stability than a commercial Pt/C,which paves the way for designing highly active and long-durability non-noble metal electrocatalysts for fuel cell.Nanosheet-like cobalt oxide(Co3O4)structures were synthesized by adjusting the urea concentration.When the urea concentration is low,rod-like Co3O4 is formed;in contrast,sheet-like Co3O4 formed.Nanosheets Co3O4 have highest surface area and largest porous structure,and lowest charge-transfer resistance,which could efficiently accelerated electron transport with the performance for OER.Meanwhile,the nanosheets Co3O4 show good performance for ORR.