Controllable Synthesis Of Ternary Non-noble Metal Sulfides And Their Catalytic Properties As Oxygen Electrode

New clean and renewable energy conversion and storage technologies including water splitting,fuel cells and metal air batteries have brought new hopes for solving the increasingly severe energy shortage and environmental pollution problems.These electrochemical driven technologies involve basic electrochemical reactions,including the oxygen reduction reaction?ORR?,hydrogen oxidation reaction?HOR?,hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?.Among these reactions,ORR and OER involve complex four-electron transfer and intermediate state conversions,which result in their sluggish kinetics,thus acting as the rate limiting step in these electrode reactions.In today's applications,the noble metals Pt-,Ir-and Ru-based materials have long been recognized as highly active catalysts for these two reactions,but their high price and limited reserves have hindered their large-scale commercial applications.Therefore,many researchers have made efforts to develop non-noble metal materials or carbon materials with high activity and stability as catalysts to replace precious metals.According to the adsorption model on the electrode surface and theoretical calculations,compared with binary metal compounds,the multivariate metal compounds can induce the alloy effect between different metals and cause the interaction of electrons between metals,which could rearrange the charge distribution,thus helping achieve the purpose of regulating each other's electronic structure and produce more reactive sites,so as to optimize their intrinsic catalytic performance.As a result,the polymetallic compounds have a potential to become efficient OER and ORR catalysts.In this thesis,the reaction factors including precursor inputs,solvent ratio,surfactant type and amount,reaction temperature and reaction time in the colloidal synthesis were accurately controlled for the preparation of high purity multicomponent non-noble metal sulfides with optimizing their content and morphology and the in situ combining with graphene oxide.By combining the characterizations,the experimental results and DFT calculations,we analyzed the relationships between the composition,morphology and structure of transition metal sulfides and their catalytic performance.Besides,zinc air battery device was used to evaluate the value of catalyst in practical industrial application,which is of great significance for the research and development of non-noble metal sulfide bifunctional electrocatalysts.This thesis has the following contents:1.In order to take advantage of the enhancement of the catalytic properties of non-precious metal compounds by alloy effect and the relatively good conductivity of metal sulphides,we synthesized the ternary metal sulfide FeNiS₂ nanoparticles for the first time by controllable one-step colloidal synthesis method,and optimized the OER activity of non-precious metal sulfides.The growth mechanism of FeNiS₂ nanosheets in the synthesis process was explored by controlling reaction time and the combining the phase and morphology characterization.After annealing treatment,it was tested as an OER catalyst.The results show that the OER catalytic properties of FeNiS₂ nanosheets were greatly improved compared with binary sulfides FeS nanosheets and Ni₉S₈ nanorods,and were superior over the commercial RuO₂.This can be attributed to the unique two-dimensional sheet nanostructure of FeNiS₂ nanosheets,which could enhance the catalytic active area and the alloy effects between Fe and Ni inducing the regulation of on each other's electronic structures,which changed the valence electron states.Our work has enhanced the intrinsic OER catalytic activity of non-precious metal sulfides through the alloy effect between metals and the construction of two-dimensional structure of materials,which could provide effective and feasible material optimization approaches for related studies in the future.2.Based on the irreversible surface self-oxidation phenomenon of FeNiS₂ in the process of OER under alkaline conditions,we revealed the impact of self-oxidation process on its catalytic properties and morphological structure using detailed characterizations.Then,we realized the in-situ combination of two-dimensional FeNiS₂ nanosheets and reduced graphene oxide?rGO?by one-step colloidal synthesis method and subsequent annealing,which successfully weakened the negative effects of the self-oxidation process on the material and achieved the further enhancement of its OER catalytic properties.In the process of OER,FeNiS₂ acted in a high potential interval,and the self-oxidation occurred on the surface to generate real active sites,which was beneficial to improve its catalytic activity.However,prolonged over-oxidation would lead to the amorphization of the material and the mass dissolution of the surface Ni and S,which was not beneficial for the stability of the FeNiS₂ in the reaction.The characterizations and electrochemical tests show that the hybrid of two-dimensional FeNiS₂ nanosheets and rGO formed interrelated three-dimensional structures and the good conductivity of rGO accelerated the transfer rate of electrons between the two parts.Besides,rGO also inhibited the dissolution of active ingredients,making FeNiS₂/rGO work steadily with little activity attenuation after a continuous 5000 CV cycles.Therefore,the method of in-situ combining with rGO proposed in this work contributed greatly to the improvement of material conductivity and stability,which greatly increased the feasibility of applying the material.3.The development and application of Zn air batteries need bifunctional non-noble metal catalysts towards OER and ORR.Two dimensional C0₉S₈ nanosheets is a kind of material with excellent ORR catalytic properties,but its intrinsic OER performance is not satisfactory.In order to enhance its intrinsic OER activity,we optimized C0₉S₈ nanosheets to generate FeCo₈S₈ nanosheets by introducing a moderate amount of Fe to substitute Co in the lattice by one-step colloidal synthesis method.The in-situ combining with rGO was also achieved through the way mentioned above?one-pot colloidal synthesis and subsequent annealing?.As a result,FeCo₈S₈/rGO was synthesized.Experimental and theoretical results show that the introduction of Fe optimized the active sites on the surface of Co₉S₈,which greatly improved the intrinsic OER catalytic activity of the material.A small amount of Fe did not affect the lattice structure of the Co₉S₈,thus,the material still maintained good ORR catalytic activity.The in-situ composite structure of FeCo₈S₈ and rGO enhanced the conductive capacity and long cycle stability of the material.By applying FeCo₈S₈/rGO as the air electrode of the Zn air battery,the battery could run 800 charge-discharge laps?-44 hours?stably at room temperature.The excellent bifunctional electrocatalytic performance of FeCo₈S₈/rGO makes it a potential substitute for noble metal catalysts and this work provides valuable information for the design and optimization of bifunctional electrocatalysts in the future.