Design,Systhesis And Electrocatalytic Properties Of Cobalt-Iron-Based Nanocatalysts

Hydrogen production from water electrolysis and Zn-air battery have become the research frontier and hotspot in the field of energy conversion and storage due to their green and efficient advantages.Three kinds of electrochemical reactions（HER,OER and ORR）are involved in water splitting and Zn-air battery,and noble metal-based catalysts are used in such reactions.Nevertheless,the scarcity,high cost and not so stability of noble metal-based catalysts restrict their large-scale application.Thus,it is necessary to design and develop of non-noble metal-based catalysts with enhanced kinetics and high catalytic activity for electrochemical catalysis.Although transition metal compounds have shown good catalytic potential,their low intrinsic activity,insufficient stability and one-dimentional catalytic site shown in electrocatalytic reactions can not be ignored.Bear these in mind,transition metal compounds were selected as research objects,and their electrocatalytic OER performance was enhanced by means of compositing with conductive matrix,micromorphology regulation,metallic element diversification,heterogeneous element doping and interface engineering.And the interrelationship among composition,crystal surface as well electronic structure and the electrochemical performances is systematically illustrated.In this dissertation,several points addressing these aspects have been done:1.Systhesis of Co₈FeS₈ based catalysts with bossted electrocatalytic OER performance（1）Carbon-based composite catalyst with heterogeneous Co Fe-Co₈Fe S₈ as main catalytic species were prepared through integrating Co Fe alloy and Co₈Fe S₈.The synergistic effect of highly conductive Co Fe alloy and highly active Co₈Fe S₈ endows the catalyst excellent OER performance.It shows an ultralow Tafel slope of 38 m V dec^-1 and a low overpotential of 290 m V was required to afford the current density of 10 m A cm^-2.（2）CNT-loaded heterostructure CoS-Co₈FeS₈ catalyst was synthesized and the microwave-assisted solvent treatment with a“bubble bursting”effect was proposed to facilitate the ordered rearrangement of its nanostructure.The findings shown that the excellent performance owing to the strong electron coupling in the interfaces of Co₈Fe S₈and Co S as well as the chemical bonding between Co S-Co₈Fe S₈nanoparticles and CNT matrix.These merits enable the catalyst excellent OER performance with an overpotential of 278 m V to achieve 10 m A cm^-2,a small Tafel slope of 49 m V dec^-1,and excellent long-term stability.（3）The catalyst Co₈Fe S₈@Fe₅C₃@C with a sandwich structure was designed and prepared.The findings shown that the unique sandwich nanostructure with Fe₅C₃ as intermediate layer will facilitate electron transfer from the core to carbon layer,which is prefer to achieve better catalytic activity.The optimal catalyst possessed an excellent OER performance with an overpotential of 275 m V to obtain 10 m A cm^-2,a small Tafel slope of 48.9 m V dec^-1 as well as excellent cyclic and microstructure stability.2.Electronic modulation of PBA derived ternary metal sulfide for bossting electrocatalytic performance（1）Reduced graphene oxide（rGO）wrapped trimetallic sulfide nanowires catalyst（Ni-Co-Fe-S/r GO）was constructed.Comparative study indicated that the unique morphology of sulfide nanowires homogeneously entangled with r GO nanosheets enables ultrafast electron transfer and ion diffusion;The Co constituent plays the role of an electron-accepting and donating site in the Ni-Fe-Co unit,and the charge re-balancing after electron transfer among Ni,Co and Fe leads to the optimum electron state distribution for OER catalysis.These merits enable the catalyst excellent OER performance with an overpotential of 251 m V to drive 10 m A cm^-2 and a small Tafel slope of 56.4 m V dec^-1.（2）CNT-based composite catalyst with heterogeneous Ni S-PBA as main catalytic species were prepared by a“partial sulfidation”strategy.Comparative study indicated that electron imbalance and partial transfer between the two species due to the stronger electron-donating behavior of Ni element in Ni S-PBA;The smallest uphill barrier was realized for Ni S-PBA and the rate-determining step became the oxidation of*OH to*O.These merits enable the catalyst excellent OER performance,featured by ultralow overpotential of 253 m V to achieve 20 m A cm^-2with a Tafel slope of 49.8 m V dec^-1.3.Systhesis of PBA derived dual-functional catalysts for water electrolysis and Zn-air battery（1）Bi-Functional OER,HER electrocatalyst was synthesized via a facile co-coordination reaction followed by aphosphorization for dual-ligand PBA precursor.The findings revealed that,compared with the monometallic Fe P or Co P,the enhanced catalytic activities of Fe Co P is mainly manifested in its free energy of H adsorption closer to zero,larger binding strength for H₂O and higher electrical conductivity.These merits enable the catalyst excellent performances for OER and HER with much lower Tafel slope of 39.6（57.1）m V dec^-1,an overpotential of 286（166）m V@10 m A cm^-2 for OER（HER）process.Being as cathode and anode catalysts for water splitting,it requires only a low voltage of 1.67 V to obtain 10 m A cm^-2 with nearly 100%faradaic efficiency.（2）Bi-Functional OER,ORR electrocatalyst（CoFe@NC-NT）featured by a nanoarchitecture of N-doped carbon nanotubes capped with carbon layer armored Co Fe alloy was facilely fabricated.The results prove that the assembly of one-dimentional CNT and core-shell Co Fe nanostructure enables ultrafast electron transfer in the catalytic process;The doped heterogenous N element in the carbon matrix enrichis the catalytic sites;The coating effect of carbon layer shell enable the catalyst superior long-term operation stability.These merits enable the catalyst excellent performances for OER with a remarkably low overpotential of 257 m V to obtain 10 m A·cm^-2 and a Tafel slope of 43.7m V·dec^-1.Meanwhile,it shows a half-wave potential of 0.74 V to obtain 10 m A·cm^-2 and a Tafel slope of 83.0 m V·dec^-1in ORR process.The Co Fe@NCNT was additionally selected as air-diffusion electrode for realizing rechargeable Zn-air battery,and the assembly device shown better performance than that of mixed catalyst（Pt/C+RuO₂）.