Synthesis and characterisation of cyclomatrix poly(organophosphazenes) / zeolitic imidazolate framework derived core@shell carbon nanomaterials for energy storage applications

The increasing demand of portable devices and electric vehicles has raised unprecedented challenges in development of high performance energy storage devices. To fulfil the growing needs for energy storage applications, a variety of methods have been investigated to produce new and functional energy materials with enhanced properties to improve the performance of Lithium-ion batteries, supercapacitors and fuel cells. Traditional anodes in Lithium-ion batteries and supercapacitors are mostly made of carbon materials such as graphite and active carbon, which highly limit the power density and cycling stability due to their low surface area and short of redox reaction sites. There are two main paths to improve the performance of electrodes, i.e., modification of traditional carbon materials or synthesis functional porous carbon nanomaterials. The current synthesis or doping technologies involve complicated processes, high energy cost and heavy chemical dosage. It is important to develop functional carbon nanomaterials via fast, energy efficient and environment friendly methods. In this thesis, a facile and rapid pathway of synthesising POP/ZIF core@shell structure was investigated. It was firstly obtained as POP/ZIF core@shell structure. The obtained core@shell structure reached a surface area of 1557 m2/g, with well distributed Zn and N, S, P, O, Cl. When applied as anode material for lithium ion battery, a stable capacity of 538 mA h g-1 was successfully reached after 250 cycles, rate stability was high as well with great capacity retention. A bi-ZIF was introduced to the core@shell structure, replacing the pure Zn ZIF-8 with a dual metallic Co-Zn bi-ZIF. The particle size of bi-ZIF can be easily tuned from 164 nm to 337 nm by modifying the ratio of the two metal, and the crystal structure of ZIFs was well preserved. The carbonised core@shell sample can reach a high surface area of 1000 m2/g. The carbonised POP/bi-ZIF with a Co : Zn molar ratio of 7.5 : 2.5 showed a capacity lower than 372 mA h g−1 after 2 cycles and poor cycling stability. The lower Co content in the carbonised POP/bi-ZIF core@shell structure benefits the electrochemical capacity and stability compared to traditional graphite. A reversed core@shell structure of POP/ZIFs was obtained in the end. The POP shell ZIF core structure can turn into hollow spheres after carbonisation. The core@shell Yutao Zhou vii structure inherit the crystalline structure from ZIFs according to the XRD results. The surface area of the core@shell structure was up to 864 m2 g-1 and 2171 m2 g-1 after carbonisation. The carbonised bi-ZIF/POP samples showed much better ORR catalysis results comparing to carbonised ZIF-8/POP samples, and reached an onset potential of 1.10 V with a half-wave potential of 0.74 V at 1600 rpm, which is even higher than commercial Pt/C catalyst. The synergistic effects between bi-ZIF and POP promoted the ORR activity performance