Controllable Synthesis Of Transition Metal Sulfides And Studies On Electrochemical Properties

To realize the sustainable development of human society,the urgent task for all countries is to get rid of the bound of fossil fuels and develop the renewable energy.Electrochemical energy storage technologies including supercapacitors and fuel cells,with the advantages of high efficiency,low cost,noise-free and non-pollution,are considered to be the most promising energy storage and conversion equipment.The electrode material and catalyst serve as the core material of supercapacitors and fuel cells,whose chemical composition and crystal structure as well as micromorphology directly determine the electrochemical properties of devices.Therefore,the key to improve the performance of the devices is to explore nanomaterials with controllable morphology and composition.In this paper,we focused on the synthesis of transition metal sulfide with high performances using simple synthesis routes based on the Kirkendall effect.The hollow porous sulfides with various structures were successfully prepared.Meanwhile,the formation mechanisms of different microstructures were investigated by tuning the synthesis conditions.Moreover,the relationship between structures and electrochemical properties was studied,which provided a theoretical basis for the development of new typical materials with high performances.The main contents were summarized as follows:Firstly,the hollow capsule-like Ni-Co sulfide electrode material was successfully prepared with ?-MnS as template adopting a cation-exchange method to achieve the control of structure and chemical composition of Ni/Co.Initially,an olive like ?-MnS nanoparticle was synthesized with manganese chloride as manganese source and sodium sulfide as sulfur source as well as trisodium citrate as weak reducing agent and structural direction agent.Afterwards,the hollow capsule-like Ni-Co sulfide was prepared with the ?-MnS as precursor by second hydrothermal reaction based on a cation-exchange reaction.The morphological evolution of the hollow Ni-Co sulfide was carefully investigated by adjusting the hydrothermal time,reaction temperature and the ratio of Ni/Co during the cation-exchange process.Due to the higher surface area and shorter ion transport path induced by porous hollow structure,the Ni1.77Co1.23S4 electrode delivers a high specific capacity of 224.5 mAh g^-1 at a current density of 0.25 A g^-1.An all-solid-state asymmetric supercapacitor assembled with a Ni1.77Co1.23S4 as the positive electrode exhibits a high energy density of 42.7 Wh kg^-1 at a power density of 190.8 W kg^-1.The fully charged as-prepared supercapacitor can light up a light emitting diode?LED?indicator for more than 1 h,indicating potential application prospects of the hollow Ni-Co sulfides.Secondly,a peanut-like MnCO₃ microcrystal with novel morphology and high surface area was firstly designed and synthesized,then a hollow peanut-like ?-MnS electrode material,which is similar to the micromorphology of peanut-like MnCO₃,was obtained using an anion-exchange method.The formation mechanism of the hollow structure and phase change were explored by tuning the reaction time in hydrothermal system.The electrochemical study shows that the hollow peanut-like ?-MnS microcrystal has high specific capacity of 257.1 F g^-1 at a current density of 0.25 A g^-1,high rate capability(the capacity retention is 66.4 % at a current density of 5 A g^-1)and long cycle stability?the capacity retention is 80.87 % after 1000 cycling?.In addition,a HPM//HMC supercapacitor assembled by the hollow peanut-like ?-MnS as the positive material and a home-made activated carbon as the negative material also exhibits high energy density of 15.5 Wh kg^-1 at a power density of 64.58 W kg^-1.Combined with their low cost,recourse abundance and environmental friendliness of the manganese element,the high performance of the hollow peanut-like MnS can boost their promising practical applications in energy storage.Finally,as a non-noble metal electrocatalyst,the ?-MnS@rGO nanorods with hollow structure was synthesized with manganese acetate as manganese source and ascorbic acid as a weak reducing agent and structural direction agent based on Kirkendall effect by a simple one-step hydrothermal synthesis strategy.The formation mechanism of the hollow nanorods was carefully explained by investigating the morphology evolution with different hydrothermal time.The electrocatalystic tests show that the ?-MnS@rGO electrocatalyst exhibits the excellent ORR activity closed to that of commercial 20 % Pt/C catalyst,methanol tolerance ability and stability,which confirms the remarkable application prospect.