Construction And Electrocatalytic Performance Of Spinel-Based Nanostructures

The development of green and sustainable energy is widely regarded as an important solution to environmental pollution and energy crises,and it is also in line with China’s dual carbon strategic goal of"carbon peak by 2030 and carbon neutral by 2060".Electrocatalytic water splitting,which contains oxygen evolution reaction（OER）and hydrogen evolution reaction（HER）,have attracted wide attention from scholars as a new energy storage and conversion system.In addition,Zn-air batteries,which includes OER and oxygen reduction reaction（ORR）have been also investigated widely.However,these three reactions including OER,HER,and ORR often require electrocatalysts with high catalytic activities to improve their performances.Currently,precious metals are commonly efficient catalysts ysued for ORR,HER,and OER,but their scarce reserves and high prices limit their wide application in the fields of renewable energy.Therefore,seeking and developing cheap and efficient catalysts for water splitting and Zn-air batteries have become the focus of research.In this thesis,spinel materials were employed as based material,and their electronic structures and exposed electrocatalytic active sites were optimized through strategies of interface engineering,anion substitution,and cation regulation,thereby enhancing their electrocatalytic activities and stabilities.Besides,through the phase and structure characterization after the reactions,the mechanisms of the catalytic process were investigated.On this basis,a series of structure-optimized spinel materials were used as multifunctional electrocatalysts for water splitting and Zn-air batteries,thus obtaining good performances.The specific research contents mainly include:Firstly,the strategies of interface engineering and anion substitution were used to optimize spinel materials as high-effiency OER and HER bifunctional electrocatalysts for water splitting.Strong electron interactions at the interface can regulate the electronic states of metal surfaces and optimize the electronic structures of materials,the Ce O_x-modified Ni Co₂O₄nanosheets array was grown on NF by a two-step synthesis method.Through the investigation of electrocatalytic mechanism,the decorating of Ce O_x which containing oxygen vacancies can contribute to the surface reconstruction of Ni Co₂O₄to the cobalt/nickel oxyhydroxide with high OER activities,which significantly improves the electrocatalytic performances.In alkaline electrolytes,Ce O_x@Ni Co₂O₄/NF exhibits superior electrocatalytic activities and stabilities,requiring the overpotentials of 238 m V for OER and 144 m V for HER to reach a current density of 10 m A cm^-2.After fabricating into a water-splitting device,Ce O_x@Ni Co₂O₄/NF shows good performances and stabilities.CoNi₂S₄ nanosheet arrays were prepared by a two-step hydrothermal method,and the effect of anion substitution on the electrocatalytic performances of spinel materials was explored through the substitution of S ions.Through structural characterization after reactions and mechanism analysis,the results exhibit that the metal ions combined with S can be reoxidized into high-valence metal oxide species with high activity during the OER process,thus promoting the OER reaction.The as-prepared catalyst exhibits the overpotentials of 230 and 193 m V to reach the current density of 10 m A cm^-2 for OER and HER reactions,indicating superior bifunctional electrocatalytic activities.Secondly,based on the unique coupling between spinel oxide particles and N-r GO,a high-efficiency OER and ORR bifunctional electrocatalyst of Zn Co Mn O4grown on N-r GO nanosheets was prepared by a two-step method and demonstrated remarkable catalytic activities for Zn-air batteries.Through the investigation of mechanism,the strong interaction between transition metal atoms from spinel oxide and N from N-r GO could improve the catalytic activities for ORR and OER.The density functional theory（DFT）calculations also show that the strong interaction in Zn Co Mn O₄/N-r GO promotes the adsorption of O₂ and H₂O,thus further enhancing the ORR and OER activities.Based on the superior OER and ORR performances,Zn Co Mn O₄/N-r GO was assembled into Zn-air batteries and showed a high power density of 138.52 m W cm^-2 as well as long-term stability for 48 h.Finally,the strategies of cation regulation and interface engineering were used to optimize spinel materials as high-effiency OER,ORR,and HER trifunctional electrocatalysts for Zn-air batteries powered overall water splitting.Based on the fact that Cr³⁺dopant is beneficial to the improvement of electronic structure and electrical conductivity,by adjusting the doping ratio of the Cr element in spinel materials,effects of the third metal doping on the electrical conductivity and electrocatalytic activities of spinel materials were discussed,thus showing superior HER,ORR,and OER electrocatalytic activities.Through the OER mechanism investigation,the generated Cr⁶⁺during OER can also stabilize the active sites,thus further improving the performance.The Co Mn_2-xCr_xO₄（x=1）catalyst shows significantly better multifunctional electrocatalytic activities than that of the Co Mn_2-xCr_xO₄（x=0,0.5,2）samples,with the half-wave potential of 0.82 V for ORR,overpotential of 290 m V for OER,and overpotential of 180 m V for HER.When used as the cathode of Zn-air batteries,Co Mn_2-xCr_xO₄（x=1）exhibits a high power density of140.26 m W cm^-2 and excellent cycle stability.In addition,the water-splitting device is also powered by above the Zn-air batteries used Co Mn_2-xCr_xO₄（x=1）.CoFe₂O₄/Co Fe-LDH/Co-NC was prepared by pyrolysis and carbonization of a Co-based organic framework（ZIF-67）.Benefiting from the synergistic effect of Co Fe₂O₄ with Co-NC with good ORR activity and Co Fe-LDH with superior OER performance,in both alkaline and simulated seawater solutions,Co Fe₂O₄/Co Fe-LDH/Co-NC shows excellent activities and stabilities in ORR,OER,and HER processes.The half-wave potential(E_1/2)of ORR is 0.87 V,and the power density of a rechargeable Zn-air battery is 147.36 m W cm^-2.Furthermore,Co Fe₂O₄/Co Fe-LDH/Co-NC electrocatalyst also exhibits remarkable properties of OER and HER,which can be powered Zn-air batteries.