Preparation And Performance Of Bifunctional Electrocatalysts Based On Metal Sulfides And Metal Borides

Zn-air batteries and electrochemical water splitting,as environmentally friendly energy storage technology and clean energy technology,have attracted wide attention.However,the too high HER overpotential and the slow ORR and OER kinetic limit the development of rechargeable Zn-air batteries and water electrolysis hydrogen production technology.Therefore,it is urgently necessary to develop highly efficiency electrocatalysts to accelerate ORR/OER kinetics and reduce HER overpotential.In recent years,carbon-supported metal sulfides and borides have been widely used as bifunctional electrocatalysts due to their abundant active sites,good synergistic effects,and excellent electrical conductivity.In this thesis,tetrazine polytriarylimidazole（CDIPT）and carbon nanotubes（CNT）were used as ligand and support,respectively,bifunctional electrocatalysts based on metal sulfides and borides were prepared by metal and heteroatom doping,and their applications in Zn-air batteries and overall water splitting were studied.The specific research contents are as follows:（1）Preparation and electrocatalytic preperties of FeS/Co₈FeS₈-NC bifunctional catalysts.First,a cross-linked polymer（CDIPT）was synthesized by Friedel-Crafts reaction of 3-（4-（4,5-diphenyl-1H-imidazole-2-group）phenyl）-1,2,4,5-tetrazine（DIPT）by using Al Cl₃as catalyst and CCl₄as both solvent and crosslinking agent.The FeS/Co₈FeS₈/N co-doped carbon（FeS/Co₈FeS₈-NC）bifunctional electrocatalyst was prepared by the co-coordination of CDIPT with Co（NO₃）₂and Fe（NO₃）₃,followed by mixing with thiourea and melamine and then pyrolysis.The FeS/Co₈FeS₈-NC catalyst showed an ORR electrocatalytic activity close to Pt/C electrocatalyst,with an onset potential（E₀）of 0.953 V and a half-wave(E_1/2)of 0.845 V.Such a electrocatalyst also demonstrated high OER activity,i.e.,the OER potential at 10 m A cm^-2(E_j=10)was1.610 V,which is superior to Ir O₂in 0.1 M KOH.The assembled Zn-air battery using FeS/Co₈FeS₈-NC as the air anode showed good performance,as evidenced by high power density(122 m W cm^-2)together with high specific capacity(838 m Ah g_Zn^-1).Such a device could be stably cycled for 900 h at 5.0 m A cm^-2.In addition,the flexible solid-state Zn-air battery based on FeS/Co₈FeS₈-NC showed a specific capacity of 768 m Ah g_Zn^-1and power density of 74.2 m W cm^-2,such a flexible solid-state Zn-air battery could be stably discharged and charged for 100 h at 1.0 m A cm^-2,confirming the excellent durability of FeS/Co₈FeS₈-NC electrocatalyst.（2）Preparation of Co Mo B/CNT bifunctional electrocatalyst and its application in water splitting.Co Mo B/CNT bifunctional electrocatalyst was prepared by using CNT as support and sodium borohydride as reducing agent,in which metallic Co and Mo compounds were reduced to transition metal borides.XRD partten of Co Mo B/CNT catalyst showed no other diffraction peaks except for the characteristic peaks for CNT,indicating that as-synthesized Co Mo B was amorphous.In 1.0 M KOH electrolyte,the Co-5Mo-B/CNT catalyst showed excellent HER and OER catalytic erformance with an overpotential(η₁₀)of 75 m V and 291 m V at 10 m A cm^-2,respectively,and the Tafel slopes were as low as 51.7 m V dec^-1and 80.2 m V dec^-1.The double-layer capacitance(C_dl)and electrochemical active surface area（ECSA）of Co-5Mo-B/CNT were 29.1 m F cm^-1and 727.5 cm²,respectively.Co-5Mo-B/CNT was used in a two electrode electrolytic cell device,which only required a voltage of1.54 V to achieve 10 m A cm^-2for overall water splitting.Co-5Mo-B/CNT also manifested an excellent stability with a current density decrease of 0.6%after chronoamperometry for 30 h.