Design,Synthesis And Electrocatalytic Performance Of Modified Carbon Supported Cobalt-Based Nanocatalysts

Exploring the electric energy storage technology of green energy vehicles has become a necessary requirement for sustainable development.Rechargeable Zn-air batteries(ZABs)are the alternative energy storage system for commercial lithium-ion batteries(LIBs),resulting from their high energy density,energy saving and environmental protection.However,the sluggish reaction kinetics of oxygen reduction(ORR)and oxygen evolution(OER)on the air electrodes severely restricts the further development of ZABs.Therefore,it is necessary to develop cost-effective bifunctional electrocatalysts to realize the commercialization of ZABs.Pt-catalysts have excellent ORR activity,while Ir-and Ru-catalysts are recognized as the OER benchmark,but they are not only expensive,scarce,but also poor in stability and selectivity of OER and ORR,which are adverse in the commercialization of ZABs.Transition metal-based catalysts,with low cost,effective corrosion resistance and stability,possess catalytic performance comparable to precious metals,which make them bifunctional catalysts and potential applications of ZABs.Hence,focusing on the above-mentioned problems,a series of modified carbon-supported cobalt-based nanocatalysts were prepared by hydrothermal and high-temperature solid-phase method,and their electrocatalytic performance were studied in this thesis.The main contents and conclusions are as follows:(1)Using glucose as the raw material,N-doped microporous carbon spheres(N-C)were obtained by NH₄Cl solvothermal method and high temperature calcination.Then N-C spheres adsorbing Co²⁺were selenized at high temperature to prepare N-doped microporous carbon ball supported orthorhombic CoSe₂ nanoparticles(o-CoSe₂@N-C).Results show that o-CoSe₂@N-C maintains the high specific surface of N-C microporous carbon spheres,providing more active sites for ORR.The electrochemical performance is significantly improved with a half-wave potential of 0.81 V,slightly worse than Commercial Pt/C.The limiting current density of o-CoSe₂@N-C possess is the same as Pt/C and reaches steady-state diffusion control earlier.Besides,the Tafel slope is 42 m V dec^-1 and the number of transferred electrons is around 3.65,indicating that o-CoSe₂@N-C has a faster ORR kinetic reaction process and the reaction is close to a four-electron mechanism.The improvement of ORR performance is mainly benefited from the fact that N-doped carbon materials have more pyridine-N and graphite-N,as well as the synergistic effect between o-CoSe₂ and N-C.(2)Using acidified Ketjenblack(AK)as the substrate modified by HNO₃,Co(AC)₂·4H₂O as the cobalt source,and urea as the precipitant,the modified carbon supported cobalt carbonate nanoparticles(Co CO₃/AK)was synthesized by hydrothermal method.Using Co CO₃/AK as a precursor,sulfur powder as a sulfur source,a high-temperature solid-phase method was used to synthesize modified carbon supported cobalt sulfide nanoparticles(CoS₂/AK).Studies have shown that CoS₂/AK has fine crystallinity and retains a large specific surface area.Besides,CoS₂ nanoparticles,highly dispersed on the surface of AK,have an average particle size of 10 nm,which ensures more active sites.Compared with Co O/AK obtained by same calcination only without sulfur powder,the results prove that CoS₂ nano particles along with AK have stronger electronic interactions,contributing to better electrocatalytic ORR activity with the half-wave current of 0.825 V,limiting current density of 5.35 m A cm^-2,Tafel slope of 36 m V dec^-1.Furthermore,CoS₂/AK still maintains 96%of the initial activity after 10 h by chronoamperometry test.The decent ORR activity of CoS₂/AK mainly benefitted from the enhanced conductivity and anchoring effect of AK,avoiding the agglomeration of CoS₂ nanoparticles.Moreover,the synergy between CoS₂ and AK optimizes the electronic structure and accelerates the electron transport.(3)Using Co CO₃/AK as the precursor and selenium powder as the selenium source,carbon-supported different crystal phases cobalt selenide nanoparticles(CoSe₂/AK)were prepared by high-temperature solid-phase method.The results show that CoSe₂ nanoparticles have fine crystallinity and larger specific surface area,providing more active sites for OER and ORR.After calcination,neither o-CoSe₂nor c-CoSe₂ have obvious agglomeration.The nanoparticles with average particle size of 10 nm are highly dispersed on the surface of AK.Electrochemical results show that o-CoSe₂ has better ORR and OER catalytic performance.Its overpotential of OER at 10 m A cm^-2 is 390 m V,which is better than that of Ir O₂.The half-wave potential of ORR is 0.84 V close to commercial Pt/C.This is mainly due to the inherent metallic properties of CoSe₂,which makes it have a richer electron cloud density.Compared with c-CoSe₂,o-CoSe₂ have more Co0(Co metal)and Co-Se covalent bonds and stronger electronic interactions with the substrate AK,which further enhances its bifunctional electrocatalytic activity.The catalyst was assembled to a Zn-air battery,and the result showed that it has better polarization performance,higher energy density and decent cycle stability than a.cathode catalyst mixed with Pt/C and Ir O₂.