Electron/ion Transport Of Two-dimensional Assembled Thin Film And Its Electrochemical Performance

The research of electron and ion transport behavior is the key of relationship between the intrinsic structure and function of materials for human beings all along.With the rapid development of nano-sized low-dimensional materials,two-dimensional assembled thin film becomes a perfect material platform for the research of electronic and ion transport behavior owing to its confined dimensionality,high specific surface area,easy regultation and many other advantages.By means of the physical and chemical modification of the two-dimensional assembled thin film,it is expected to realize the effectively regulation of electron and ion transport behavior,and finally realize the optimized function of the two-dimensional assembled material.In this thesis,the author based on the electronic and ion transport characteristics as a key of the study,bringing effective modulation of the relevant two-dimensional assembled film by electrical,magnetic,optical behavior,and ultimately triggering novel electronic transport properties and excellent electrochemical performance.In particular,the author used valence regulation,oxygen defects,polymer assembly and other methods to significantly enhance responsiveness and functionality of two-dimensional assembled thin film.This thesis provides a new idea for the modulation strategy of electrons and ion transport behavior for two-dimensional assembled thin film and further realizes the excellent smart response and electrochemical performance.The details are included as follows:1.So far,electronic state transition especially for metal-insulator transition（MIT）offers electron transport characteristics useful in intriguing energy applications and smart devices.But to-date very few simple metal oxide possessed electronic transition near room-temperature.The author regulated valence-state of δ-MnO₂ and first experimently realized Mn（Ⅲ）-O-Mn（IV）structure with double-exchange effect in 2D nanomaterials,bringing a metal-insulator transition near room-temperature as well as novel electron transport behavior.The precise local structure of Mn（Ⅲ）-O-Mn（Ⅳ）in MnO₂ was also analyzed by synchrotron X-ray fine absorption spectroscopy.As expected,double-exchange in 2D MnO₂ exhibits an MR value up to-11.3%（0.1 T）at 287 K,representing the highest negative magnetoresistance values in 2D nanomaterials approaching room-temperature.Also,MnO₂ nanosheet displays infrared（IR）response of 7.1%transmittance change from 270 to 290 K.Dimensional confinement of double-exchange structure would realize the modulation of electronic state,promising novel magnetotransport properties and sensitive response for smart devices.2.Nowadays,the design of aligned ion migration channels is the key of improving the intrinsic capacitance for constructing high-performance energy storage devices.Zwitterionic PPDP gel electrolyte has a combination of advantages of aligned ion migration channel,robust water retention ability and reasonable mechanical strength,which is beneficial to the two-dimensional assembled thin film to achieve good electrochemical performance.The author enhances the capacitive behavior of 2D graphene film ’via applying a novel zwitterionic PPDP gel electrolyte.Specifically,the ion migration channel could be formed along the aligned zwitterionic side groups within the PPDP gel electrolyte induced by the external electric field,which is benefical for ion transport of salt in gel electrolyte.Synchrotron radiation soft X-ray near-edge absorption spectroscopy confirmed that zwitterionic side groups could be aligned by external electric field.Meanwhile,each zwitterionic group is highly hydrated by the surrounding eight water molecules with robust water retention ability,greatly improving the electrochemical performance of 2D graphene based solid-state supercapacitors.When applying PPDP as gel electrolyte,the as-fabricated 2D graphene-based solid-state supercapacitor reaches a volume capacitance of 300.8 F·cm^-3 at 0.8 A·cm^-3,with a rate capacity of only 14.9%capacitance loss when the current density increases by the factor of 25,recording the best value amongst the previously reported graphene-based solid-state supercapacitors.Moreover,the high water retention ability of zwitterionic groups brings a robust cyclability,the capacitance remaining 103%retention after undergoing 10000 charge-discharge cycles.The construction of aligned ion migration channels in electrolytes provides a new idea for the design of novel energy storage devices based on two-dimensional materials.3.The lower conductivity of manganese dioxide itself hinders its practical use in electrocatalysis.Inspired by the previous work,the author regulated oxygen defect engineering of 2D MnO₂ for enhanced electrocatalytic performance.In our case,we successfully modulated the electrical behavior of δ-MnO₂ nanosheets by introducing oxygen defect with improved the conductivity and reduced the band gap,and finally obtained the improvement of oxygen reduction performance.We also used the synchrotron radiation soft X-ray near-edge absorption spectroscopy to confirm the process of electron injection when oxygen defect generated.The partial electrons of Mn 3d shifted to the O 2p orbital,resulting in a significant modulation effect.The O-2D MnO₂ sample with oxygen defect achieved a limiting current density of-2.68 mA·cm-2 at 1600 rpm,which was significantly higher than-1.64 mA·cm-2 of the P-2D MnO₂ sample without defect.Also,the half-wave potential of O-2D MnO₂ sample is 12 mV higher than that of P-2D MnO₂ sample.We anticipate that the regulation of the electronic structure of the 2D material through oxygen-deficient engineering can be an effective way to achieve enhanced oxygen reduction properties.