Constructing Multifunctional Nanoscale Electrode Materials By Selectively Etching Of Carbides With Chlorine

To cope with the energy and environmental crisis,clean and highly efficient electrochemical energy conversion and storage devices?such as fuel cells,lithium ion batteries,metal-air batteries,etc.?have been widely concerned by researchers.However,the ration design,construction and controllable synthesis of high performance electrode materials are facing great challenges.This thesis focuses on constructing a class of multifunctional nanoscale electrode materials fabricated by selectively etching of carbides with chlorine and thus achieves a series of great progress:?1?The thesis successfully prepared B and Cl dual-fuctionalized carbon nanotubes?BClCNTs?via chemically tailoring boron carbide with Cl₂.Accompanied by controlling partial extraction of B atoms from B₄C with Cl₂,the residue atoms in the system can self-organize into nanotube microstructure with B-C-Cl moieties.The amount of doping B and Cl atoms can be tuned in terms of altering chlorine-to-carbide molar ratios.Specifically,the produced BClCNTs demonstrate a high activity as well as stability for oxygen reduction reaction?ORR?.Theoretical calculations suggest that B-C-Cl moiety is the key to excellent ORR activity.The presented strategy for low-cost and controlled preparation of heteroatom doped CNTs offers prospects in multitudinously fascinating applications.?2?The thesis reports a generic chlorine selectively etching technique for a universal transformation of abundant 2D metal carbides?M_xC_y,e.g.Cr₃C₂,Mo₂C,NbC and VC?to 2D M-self-doped graphene?MG?.The as-obtained MG endows a transparent sheet architecture of one to four atomic layers.Simultaneously,MG with different M amount is synthesized by tuning the chlorination parameters.Among them,the novel and representative Cr-self-doped graphene with optimal Cr amount?4.81 at%?demonstrates the outstanding electrochemical performance.It presents an energy density of 686 Wh kgelectrode^-1 and a power density of more than 391 W kgelectrode^-1 as anode material of Li ion batteries,and four-fold activity against the commercial iridium oxide electrode towards oxygen evolution reaction?OER?as well as a comparable oxygen reduction reaction?ORR?performance to the commercial platinum catalyst.Moreover,this method overcome the problem of bigger metal atoms doping into carbon six-membered rings in graphene and thus is readily scalable to produce MG electrode materials on industrial levels.?3?The thesis conducts theoretical calculations which show that high-index?222?facets of TaC are dramatically more active than its other facets towards hydrogen evolution reaction?HER?.However,due to their high surface energy,high-index facets easily evolve into relatively low-index facets and thus causes a fabricated challenge.Therefore,the thesis develops the incomplete chlorination towards bulk TaC to in situ obtain the carbon-protected high-index?222?faceted tantalum carbide nanocrystals?TaC NCs@C?.Interestingly,benefiting from transition zones between in situ formed carbon layers and?222?facets,the evolution of high-index?222?facets to lower index facets can be prevented during preparation and electrochemical reaction.When evaluated as HER catalyst,TaC NCs@C presents a low overpotential of¹46 mV at 10 mA cm^-2,large exchange current density of 9.69×10^-2 mA cm^-2 and outstanding long-term cycling performance.To the best of our knowledge,this HER performance is far preferable to that of the reported group V metal carbides-based catalysts.?4?Contrary to popular belief on exaction of metal atoms from carbides with Cl₂,we do exactly the opposite to remain metal covers on the carbide by partially removing carbon atoms.Based this design concept,we create WC covered by metal W?WC@W?via chemically tailoring WC with chlorine and adjusting the experimental configurations.As a typical HER electrocatalyst,WC@W exhibits onset overpotential as low as 31 mV,accompanied by low overpotential of 159 mV at 10mA cm^-2 and excellent stability.When coupled to a p-type Si semiconductor that harvests red photons in the solar spectrum,the current density of WC@W reaches 16mA cm^-2 at the reversible hydrogen evolution potential.Our results provide guidance for paradigm shift of designing a class of advanced carbide-based catalysts.