Preparation And Electrochemical Performance Of Oxygen Electrode Catalysts

The increasing demand for clean energy and environmental quality have promoted the global exploration of alternative and sustainable energy conversion systems,such as water electrolysis,metal-air batteries and fuel cells and so forth,but the sluggish oxygen reduction reaction and oxygen evolution reaction?ORR/OER?kinetics and high overpotential greatly limit the large-scale application of these technologies.In spite of Pt-based catalysts and IrO₂/RuO₂ are very active for ORR and OER respectively,their poor bifunctionality,inferior stability,the high cost together with the scarcities of noble metals still significantly hinder their be an ideal bifunction oxygen electrode catalyst.Therefore,it is quite necessary to search for alternative efficient and durable non-precious metal bifunctional oxygen catalysts.In order to prepare high-efficiency and low-cost oxygen electrode catalysts,a series of transition metal-supported carbon-based catalysts were synthesized by simple pyrolysis method.The structure and electrochemical properties of the catalysts were systematically analyzed by combining modern spectroscopic analysis techniques and electrochemical methods.The research content is as follows:?1?two-dimensional g-C₃N₄ sheet not only situ coupled with metallic species to form core-shell structure,but also induced to fabricate nitrogen-doped graphene during calcination process.The Co@NC catalyst was constructed via facile annealing procedure using glucose,dicyandiamide,Co?NO₃?₂·6H₂O as carbon,nitrogen,and cobalt sources,respectively.The physical characterizations demonstrate that the Co@NC catalyst possesses high specific surface area with plentiful pore structure,special core-shell structure with the metal core and carbon layer shell,high content of N doping?8.8 at.%?,and some Co-N-C bond.The Co@NC catalyst shows superior ORR performance with high positive onset potential E_onset(j=-0.1 mA cm^-2)of 1.0 V and half-wave potential E_1/2(j=-3 mA cm^-2)of 0.84 V.Furthermore,the polarization curve has hardly changed through 8000 sweeping cycles and the Co@NC catalyst exhibits merely attenuation with a current loss of 0.4%after 1800 s,suggesting a better stability for Co@NC catalyst.?2?The Co@N,S?two?-Kb catalyst as a bifunctional electrocatalyst for ORR/OER was constructed via facile annealing procedure using ketjenblack,melamine and thiourea,Co?NO₃?₂·6H₂O as carbon,nitrogen and sulfur,cobalt sources,respectively.The physical characterizations shows that the metal Co particles are coated in the carbon layer and uniformly distributed,and the heteroatoms N and S are successfully incorporated into the carbon skeleton with high amount of pyridine-N,thiophene-S.The oxygen electrode activity?E=E_OER(j=10 mA cm^-2)-E_1/2(j=-3 mA cm^-2)is 0.85 V.Furthermore,the polarization curve of Co@N,S?two?-Kb has hardly changed through 8000 sweeping cycles?negative shift 20 mV of half-wave potential for Pt/C catalyst?and the Co@N,S?two?-Kb catalyst exhibits slightly attenuation with a current loss of 1.4%after 1800 s?Pt/C of 17%?,suggesting outstanding catalytic stability for Co@N,S?two?-Kb catalyst.?3?The FeNi@NC catalyst was synthesized by pyrolyzing glucose,dicyandiamide,FeCl_3·6H₂O,and NiCl₂·6H₂O respectively.Physical characterizations demonstrate that the core-shell sructure FeNi nanospheres with a mean diameter of 11 nm are uniformly loaded on the NG,and high content of pyridine-N and graphite-N,abundant porous structure and high specific surface area provide multiple active sites for the ORR/OER and charge/mass transport channels.The oxygen electrode activity?E=E_OER(j=10 mA cm^-2)-E_1/2(j=-3 mA cm^-2)is 0.82 V.No change was observed for FeNi@NC after 8000 continuous cycles in 0.1 M KOH,demonstrating the excellent stability of FeNi@NC.