Construction Of Cobalt-based Carbon Nanotube Catalysts And Their Application In Oxygen Electrode

Oxygen electrocatalysis,namely of the oxygen evolution reaction?OER?and the oxygen reduction reaction?ORR?,governs the performance of numerous electrochemical energy systems such as reversible fuel cells,metal–air batteries,and water electrolyzers.However,the sluggish kinetics of these two reactions and their dependency on expensive noble metal catalysts?e.g,RuO₂,IrO₂,and Pt?prohibit the sustainable commercialization of these technologies.In recent years,nanoncarbons supported various non-noble metals catalysts are intensively explored as oxygen electrode electrocatalysts.This thesis work focused on developing high efficient Co-based carbon nanotubes composites and investigating their electrocatalytic properties for oxygen electrode reactions.The main contents are summarized as follows.1.Ultrasonication-assisted ultrafast preparation of multiwalled carbon nanotubes/Au/Co₃O₄ catalyst as superior anode materials for oxygen evolution reactionWe present a novel ultrasonic approach to synthesize a uniformly loading CNTs/metal/transition metal oxide(CNTs-Au@Co₃O₄tubular composite.The preparation process is simple,mild and ultrafast,noteworthy,it does not require any surfactants or further heat treatment.In alkaline media,CNTs-Au@Co₃O₄ catalyze OER with an onset potential of 1.50 V _vs.RHE and overpotential only of 350 mV to achieve a stable current density of 10 mA cm-2 for at least 25 h.Besides,we investigate the effect of different electronegative metals on OER activity.The result shows that the highest electronegativity metal Au would accelerate the formation of the active centers CoIV species in OER.Moreover,the mechanism of OER on CNTs-Au/Co₃O₄ surface have been proposed.2.Coaxial ultrathin Co_1-yFe_yO_x nanosheet coating on carbon nanotubes for water oxidation with excellent activityWe fabricate a tubular iron-group binary metal nanosheet coaxial coating on crbon nanotubes material,and adequately discuss the effect of design,preparation,structure and other factors on OER activity.The research results are as follows:?1?coaxial ultrathin nanosheet exposes more electrochemical active sites than their bulk counterparts,resulting in a reduced diffusion path and an enhanced contact area with the solution;?2?the Fe dopant in our catalysts can play a partialcharge-transfer-activation?PCTA?effect,resulting in enhanced the oxidation ability of CoIII/CoIV and increased the OER kinetics.In alkaline media,the obtained optimal hybrids Co_0.8Fe_0.2O_x/CNTs25 wt% catalyze OER with a very sharp onset potential?1.45 V vs.RHE?and an exceptional over-potential?280 mV at 10 mA cm-2?for more than 14 h.Remarkably,the excellent activity of Co_0.8Fe_0.2O_x/CNTs25 wt% for the OER is superior to commercial RuO₂ and many other highly active precious-metal/transition-metal catalysts reported to date.3.Self-assembly of cobalt-centered metal organic framework and multiwalled carbon nanotubes hybrids as a highly active and corrosion-resistant oxygen catalystMetal-organic frameworks?MOFs?are a new class of crystal porous materials consisted of metal ions or clusters coordinated to organic ligands,which possess unique properties such as diverse skeletons,well-defined pore sizes,large surface areas,large pore volume and high thermal/chemical stability.In this work,we introduce a cobalt-containing zeolitic imidazolate framework?termed CoMOF?and present a facile self-assembly approach to synthesize a high active and strong durable Co-MOF@CNTs bi-functional catalyst for OER and ORR.The intertwined CNTs are successfully inserted in Co-MOF polyhedral and became an integral component of the Co-MOF framework,which not only effectively improve the conductivity but also can sustain the harsh oxidative environment of water oxidation without carbon corrosion.The hierarchical structured hybrid exhibites comparable OER and ORR catalytic activity to RuO₂ and 20 wt% Pt/C catalysts and superior stability.