| Prussion blue analog is one of the most important coordination polymers. Similar to other coordination polymers, Prussion blue analogs show us huge BET surface, high porosity, and the micropores are completely regular and highly designable. Prussion blue analogs have two different kinds of position for transition metal ions, one coordinated with carbon and the other connected with nitrogen. As both the two positions can be filled with different transition metal ions, Prussion blue analogs have rich species diversity, and also functional diversity. In the area of electrochemical capacitors, scientists have tried numerous nanocomposites or nanostructures, especially, the1D CNTs and2D graphene based metal oxides composite materials showed us excellent performance. But from the practical perspective, those complexes stem from costly elaborate synthesis, and still their energy density and power density are not large enough, so they can hardly meet the increasing demands in industry. In this article, we designed a new kind of nanocomposite materials, using metal hexacyanoferrate as core and developing a shell of metal oxide, to utilize the large energy density of the porous framework structure and fast charge/discharge property of metal oxide (mostly MnO2), and realize better electrochemical performance. We discussed two different composite structures, the core/shell structure and dual-layer structure; each of them have strengths, and could realize our original expectation.Noble metals are the most common catalyst with excellent activity and stability. However, the fact that noble elements are rare and expensive limits their large-scale usage. Usually, using the nanoscale noble metals could reduce the noble metal usage, and also improve the catalytic activity, but even though the nanoparticals are down to the size of2run, still about half of the noble atoms are buried. Recent concept of single atom catalyst provide us the new idea:we can replace the transition metal ions in metal hexacyanoferrate with noble metal ions, thus the noble metals ions could sufficiently contact with reactants, realizing the maximum utilization of noble elements.Based on the above ideas, this dissertation mainly contains the following aspects:1. A manganese hexacyanoferrate and manganese dioxide composite prepared by a simple co-precipitation technique was tested as possible electrode material for an electrochemical supercapacitor. A new method we called "Deep electro-oxidation" applied on the electrode not only kept the cycling stability by converting the Mn (II) to Mn (Ⅲ) in MnHCF, which can solve the instability of K2Mn2+Fe2+(CN)6, but also generated thicker MnO2layer on the surface of MnHCF particles. Interesting electrochemical behavior has been observed in an environmentally friendly electrolyte (0.5M Na2SO4) and an average capacitance as high as225.6F g-1was reproducibly obtained within a potential window of1.3V (voltage range-0.2to+1.1V vs. Ag/AgCl) using a sweep rate of5mV s-1. Such a wide potential window leads to a much higher energy density (74.5Wh kg-1at current density of0.5A g-1) than most reported MnO2electrodes, successfully resolves the low energy density problem of hodiernal supercapacitors. The two conponents of MnHCF and MnO2in DOME both contribute to the enhanced capacitance and high energy density, reflected on the integrated CV curves and charge/discharge curves. In addition, as revealed by XPS characterizations, oxidation state of manganese in MnHCF remains unchanged during the redox switching, the electrochemical capacitance mainly comes from the conversion between Mn (Ⅳ) and Mn (Ⅲ) in manganese dioxide and the conversion between Fe (Ⅱ) and Fe (Ⅲ) in MnHCF.2. We have rationally designed and synthesized a hierarchical dual-layer composite structure constructed by ultrathin plate-like MnO2coated NiHCF local area three dimensional network. This interesting functional nanostructure simultaneously integrates the structural and compositional design rationales for composite cathode materials based on the complementarity of reticular battery-type MOF structure and high power pseudo-capacitive MnO2layer, and the synergy effect (ICE effect) of this kind of dual-layer structure. When evaluated as a cathode material for supercapacitors, the as-prepared NiHCF@MnO2electrode manifest enhanced specific capacitance, high energy density and excellent cycling performance. The manufacture strategy demonstrated here is simple and versatile for the preparation of other dual-layer composite electrode. What’s more, the ICE effect of dual-layer structure comes to light for the first time with a guidance function for the future studies.3. Through careful experimental studies, we have demonstrated that a variety of noble elements, including Pd, Pt, Au and Ag, can sneak deep into the pre-prepared metel hexacyanoferrate and successfully take the place of some iron ions through a solution-mediated reaction. Those noble elements doped metel hexacyanoferrate are: Pd-MnHCF, Pt-MnHCF, Ag-MnHCF, Au-MnHCF, Pd-CoHCF, Pt-CoHCF, Ag-CoHCF, Au-CoHCF, Pd-NiHCF, Pt-NiHCF, Ag-NiHCF, and Au-NiHCF. The whole process is operated through a mild hydrothermal method or under room temperature without disrupting the tunable, open framework structures. In addition to the good compatibility of Prussian-blue type coordination polymers to access a series of noble metals and the convenient, mild approach, various aspects of nanostructure (e.g., size, morphology, and hollow structure formation) can be influenced by controlling critical aspects of the metal hexacyanoferrates precursors. The diversity in noble elements, and their uniform distribution throughout the open framework structure, together with highly controllable nanostructures will no doubt help to access a wide range of high performances, especially in areas of highly stable single-atom catalysis, gas storage. Along with introducing a series of noble elements mentioned above, this doping strategy also shows potential of bringing in a broad range of rare earth elements, which may help to make efficient T1and T2contrast agents for MRI. |
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