Preparation And Electrochemical Performance Of Non-noble Composites As Oxygen Catalysts

Oxygen catalysts are of critical importance for the commercialization of fuel cells,metal-air batteries and electrochemical water splitting.Pt and its alloys have been widely applied as efficient oxygen catalysts for the oxygen reduction reaction（ORR）,while RuO₂ and IrO₂ have been employed as efficient oxygen catalysts for the oxygen evolution reaction（OER）.In recent years,various oxygen catalysts such as metal/metal oxide and carobn composites have been developed as alternatives.This thesis focuses on the development of non-noble composite oxygen catalysts with high catalytic activity and stability.We prepared perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ（BSCF）and carbon nitride（g-C₃N₄）composite,and investigated them as oxygen catalysts;CoFe nanoalloy particles encapsulated in nitrogen doped carbon and nitrogen-doped carbon nanotubes（CoFe@NC-NCNT-H）were facilely synthesized by pyrolyzing Prussian blue analogue precursor（i.e.Fe₃[Co（CN）₆]₂）at a temperature of 600°C.The main results are summarized as follows:1.A non-precious graphitic carbon nitride constructed perovskite composite as an efficient electrocatalyst for oxygen reduction and evolution reactionsIn this work,we construct a non-precious and environmentally friendly composite from graphitic carbon nitride and perovskite,BSCF,g-C₃N₄ and Vulcan carbon（VC）（labeled as BSCF/g-C₃N₄-VC）,as an efficient bi-functional electrocatalyst for ORR and OER in alkaline media.We overcome major limitations of perovskite-type BSCF and g-C₃N₄,i.e.,the relatively low specific surface area(⁰.27 m² g^-1)of the former and the extremity low electronic conductivity of the latter,by ultrasonically mixing BSCF,g-C₃N₄ and VC to form a composite with a relatively high specific surface area(¹24.7 m² g^-1).Through this simple yet effective strategy,BSCF/g-C₃N₄-VC exhibits higher ORR and OER activities than the individual g-C₃N₄and BSCF,and also higher than the composites of g-C₃N₄-VC and BSCF/VC,respectively.The significantly improved electrocatalytic activities can be attributed to various factors including the utilization of conductive and high-specific-surface-area VC as a support via enhancing the electron transfer efficiency,and the synergy between g-C₃N₄,BSCF,and VC.Our results suggest that using nitrogen-rich carbon materials（e.g.g-C₃N₄）with perovskite-type oxides（e.g.BSCF）is a promising route to develop highly efficient oxygen catalysts.2.Prussian blue analogue derived CoFe@nitrogen-doped carbon as efficient bifunctional oxygen catalystIn this work,CoFe nanoalloy particles encapsulated in nitrogen doped carbon and nitrogen-doped carbon nanotubes（CoFe@NC-NCNT-H）are facilely synthesized by pyrolyzing Prussian blue analogue precursor（i.e.Fe₃[Co（CN）₆]₂）at a temperature 600°C.Such low temperature pyrolysis affords the hybrid mesoporous material with a high level of nitrogen content（¹0%）and a relatively high specific surface area(²10.5 m² g^-1),capable of providing more active sites and enhanced mass transport.In addition,CoFe nanoalloy particles facilitate the formation of a graphitic structure of carbon.The formed graphitic structure can lead to an enhanced conductivity,which favours charge transfer.Accordingly,Co Fe@NC-NCNT-H shows excellent ORR and OER electrocatalytic activities in both alkaline and acid electrolytes.In the alkaline solution,CoFe@NC-NCNT-H not only shows highly efficient OER activities with a low onset potential of¹.35 V,but also requires a small overpotential of³80 mV to reach 10.0 mA cm^-2 current density.As for ORR,CoFe@NC-NCNT-H offers competitive current densities with a direct four-electron reaction pathway,and more importantly it exhibits improved stability compared with commercial Pt/C.In the case of the bifunctionality for both ORR and OER,an extremely low potential difference of⁰.87 V between ORR at-3.0 m A cm^-2 and OER at 10.0mA cm^-2 is achieved,superior to commercial Pt/C and RuO₂.The facilely prepared low-cost CoFe@NC-NCNT-H with high bifunctional performance and stability promises great potential in replacing the precious oxygen catalysts.