Controllable Preparation Of MOF-derived Hierarchical Structure Materials And Their Application In Aqueous Energy Storage

In recent years,the extensive use and non-renewability of fossil fuels have caused serious environmental and energy problems.The development of efficient,green,safe and low-cost energy storage devices is the key to alleviate the problem.Aqueous energy storage devices,such as supercapacitors and aqueous zinc ion batteries,have attracted much attention due to their high performance,low cost and high safety.However,they still have shortcomings,such as the energy density of supercapacitors remains low and the stability of fast charging/discharging is poor.The capacity of aqueous Zn/MnO₂ batteries is low and the long cycle stability is poor,etc.So their practical application in large-scale energy storage equipment is severely limited.Electrode material is a key factor to determine the performance of energy storage devices.At present,electrode materials have the following problems:1)single component:limited voltage range,conductivity,and dispersibility;2)generally bulk powder:small specific surface area（less exposed active sites）,low porosity（slow ion transmission rate）.These problems severely limit the improvement of their energy storage performance.Multi-component hierarchical structure electrode materials can effectively solve the above problems:1)Multi-component:the synergistic effect between different components can make up for the deficiency of a single component and improve electrochemical performance;2)Hierarchical structure:large specific surface area and porosity can effectively increase the contact area between the active material and electrolyte,promote the diffusion of electrolyte ions,and thus improve the utilization of active materials and the rate of electrochemical reactions.Therefore,it is an effective way to improve the performance of electrochemical energy storage devices by designing the structure and composition of electrode materials reasonably and preparing multi-component electrode materials with hierarchical structure.And it is also an important research direction of current energy technology.This thesis focuses on the rational design,controllable synthesis and energy storage properties of MOF-derived transition metal based electrode materials for aqueous energy storage devices.The research contents are shown as following:1.A simple,easy-to-operate self-sacrificial template method was developed to prepare Ni（OH）₂-MnO₂/C electrode materials with hierarchical structure assembled from two-dimensional nanosheets.The composition and structure of Ni（OH）₂-MnO₂/C were controlled by adjusting synthesis parameters.The effect of temperature and time on the composition and structure of Ni（OH）₂-MnO₂/C,the structure-activity relationship between structure and properties,and the energy storage mechanism were studied.The supercapacitor performance tests show that the composite has high specific capacity(862.0 g^-1 at 2.0 A·g^-1),excellent rate performance(574.0 g^-1 at 40.0 A·g^-1)and long-term stability(retain 87%of the capacity after 10,000 cycles at 2.0 A·g^-1).In addition,an all-solid-state symmetric supercapacitor device was fabricated based on Ni（OH）₂-MnO₂/C composites,which exhibits high volume power density(0.672 mW·cm^-3)and energy density(0.027mW·cm^-3)as well as good flexibility and long-term stability（10000 cycles）.These excellent peformances are attributed to three-dimensional hierarchical structure and multi-components,which makes the electrochemical reaction more efficient and enhances its energy storage performance with synergistic effect of double-layer capacitance and pseudocapacitance.2.Based on the above strategy,in order to further improve the energy density and fast charge-discharge performance of supercapacitors,we have developed a simple,efficient and universal method with the help of"external template method"and"sol-gel"technology.The reduced graphene oxide/MOF-derived metal oxide（rGO/M_xO_y）composite aerogels with hierarchical structure were fabricated.The structure of rGO/M_xO_y aerogels were controlled by adjusting synthesis parameters and the effect of the kinds and proportions of raw materials on the composition and structure of the products and the universality of the preparation method were studied.The supercapacitor performances were tested and found that the electrode material has high energy density(79.2 Wh kg^-1 at 405.0 W·kg^-1)and long-term stability at high current density(5,000 cycles at 20.0 A·g^-1)compared with the Ni（OH）₂-MnO₂/C composites,besides high specific capacitance(869.2 F·g^-1 at 1.0 A·g^-1)and power density(8010.0 W·kg^-1 at 25.8 Wh kg^-1).All-solid-state supercapacitor devices based on this electrode material were fabricated and also exhibit excellent electrochemical properties,especially high energy density and cycle performance at high rate.These excellent performances can be cotributied to introduction of the external template（grapheme oxide）and construction of the three-dimensional porous aerogel,which made the electrode material better conductivity,larger specific surface area and more active sites.These advantages greatly improve the utilization ratio,reaction rate and stability of the electrode material.3.The above-mentioned"external template"method was extended to the preparation of ZnO/MnO₂/C@CC free-standing cathode materials with array structure.The formation mechanism and energy storage mechanism of ZnO/MnO₂/C@CC compositeare were studied and discussed by in-situ/ex-situ technologies.It was found by testing zinc storage performance that the cathode material has high specific capacity(349.4 mAh·g^-1 at current density of 0.5 A·g^-1),high rate performance(136.6mAh·g^-1 at current density of 4.0 A·g^-1)and long cycle stability(175.7 mAh·g^-1 at current density of 2.0 A·g^-1 after 1100 cycles and retention rate of 78.7%).These excellent properties are mainly attributed to the large specific surface area and porosity of the array,which exposes more active sites and shortens the ion transport path.At the same time,the composite material can be directly used as free-standing electrode,which avoids effectively the use of binder.These features greatly improve the utilization of the active material and the reaction rate,thus improving its energy storage performance.In addition,the existence of carbon coating can greatly alleviate the structure destruction of MnO₂ during Zn²⁺insertion/extraction process,thus improving the stability of electrode materials.