Metal Organic Frameworks Based Composite Materials for Applications in Electrochemical Energy Storage

基於金屬有機框架複合材料的電化學能源儲存應用

Student thesis: Doctoral Thesis

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Award date11 Jul 2019

Abstract

In modern society, energy storage has become a significant worldwide theme as gargantuan fossil fuel consumption. Especially, the electrochemical energy storage devices that can store/use energy more effectively.

Electrochemical energy storage systems available such as batteries and supercapacitors (SCs) have been developed during the past few years. Although lithium ion batteries (LIBs) have widely dominated in portable electronics and emerging electric/hybrid vehicles and now are considered as the possible choice for future electric vehicles and grid-scale energy storage systems, the increasing concerns about limited lithium resources, high-cost, and safety issue strongly limit their further development for large-scale applications. Furthermore, there is increasing demand for higher energy density and power density based on stable performance and cycling life. To solve these problems, developing new electrode materials with special architectures have been demonstrated to be an efficient approach to further improve the cycling performance of batteries, due to their enhanced destruction-resisting ability during the reaction-induced contraction and expansion. Metal-organic frameworks (MOFs), a new class of materials with diverse structures and tunable properties, have drawn tremendous attention for a broad range of applications including gas storage, catalysis, magnetism, solar cells etc. Their exceptionally well-ordered pores and variable sites for redox reactions make them intrinsically suitable as electrode materials for electrochemical energy storage devices. However, most MOFs lack sufficient electrical conductivity and mechanical flexibility. What’s more, MOF-derived structures are usually in powder forms, which include the unwanted polymer binder and carbon additive. In electrochemical reactions, these “dead mass” can block the active sites, and result in poor capacitance performance. Hence, using appropriate carbon-based substrates are likely to improve MOF’s electrochemical performance due to enhanced conductivity and structural stability during electrochemical processes.

Based on the above points, we firstly focus on developing a novel structure of electrode material based on flexible and highly conductive two dimensional (2D) Co-MOF/reduced graphene oxide (rGO) paper through the electrostatic self-assembly of intrinsically electronegative GO sheets and electropositive 2D-CoMOF sheets combined with subsequent reduction processes in chapter two. As a result, the as-prepared Co-MOF/rGO hybrid papers not only show alternating layer structures of rGO and MOF sheets, but also maintain high electrical conductivity (0.32 Ω cm), excellent flexibility and mechanical properties with a Young's modulus of 34.4 GPa and a tensile strength of 89.9 MPa. The area capacitances of Co-MOF/rGO paper-based symmetric SCs are determined to be 656.6 F cm-2 at a current density of 1 mA cm-2 with GO addition amount of 40 wt%. The symmetric SCs possess a maximum area energy density of 11.7 μWh cm-2 and a power density up to 986 μW cm-2. After 2600 cycles, the SCs experience capacitance reduction but still keep 92.5% of the initial capacitance.

Secondly, a one-for-two strategy is introduced to construct the other types of porous electrode via one MOF derived synthesis route with simply changing in cobalt metal ion precursors into nickel. Similarly, the electrochemical performances of the as-prepared 2D-Ni-MOF/rGO hybrid papers were also evaluated in a two-electrode coin cell in 1 M sulfuric acid (H2SO4) aqueous electrolyte. The area capacitances of Ni-MOF/rGO paper-based SCs are measured to be 570.8 F cm-2 at 1 mA cm-2. The Nyquist plots showed the intrinsic resistance of Ni-MOF/rGO paper electrodes is 11.2 Ω, which is larger than that of the Co-MOF/rGO electrodes (8.4 Ω). Moreover, the Ni-MOF/rGO paper-based SCs exhibit relatively large energy density (4.96 μWh cm-2) and power density (800 μW cm-2). In addition, a flexible and editable asymmetric all-solid-state supercapacitor using the N-M/G-40 negative electrode, C-M/G-40 positive electrode and a PVA-H2SO4 electrolyte was assembled. As a consequence, the asymmetric SCs can yield an outstanding volumetric energy density of 1.87 mWh cm-3 and an ultrahigh volumetric power density of 250 mW cm-3. More importantly, the all-solid-state asymmetric SCs offers high editability and bending-tolerant property, and performs very well under various severe conditions, such as being seriously cut, bent, and heavily loaded.

And for further improving the electrochemical performance of MOF-based SCs device, a 2D cross-linked paper structure based on 1D-Ni-MOF ribbons and 2D-rGO as cathode, zinc metal as anode and ZnSO4 as aqueous electrolyte were assembled to construct the Zn-ion hybrid supercapacitors (ZHSCs). Reversible ion adsorption/desorption on rGO/1D-NiMOF cathode and Zn2+ deposition/stripping on Zn anode enable the ZHSCs to repeatedly and rapidly store/deliver electrical energy, meanwhile, reversible pseudo-capacitance reaction occurring in weak acidic ZnSO4 solution through active 1D-NiMOF results in excellent capacitance performance for overall capacity of 146.1 mAh g−1, a very large energy output of 141.03 Wh kg−1 (corresponding to an energy of 99.95 W kg−1) and an excellent cycling stability with 98% capacity retention over 2000 cycles. The safe, high-rate and long-life ZHSCs are believed to provide new options for next-generation energy storage devices.