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Title: Graphene-reinforced MOFs-derived nanocomposites for electrochemical applications
Author: Deng, L.
Awarding Body: University of Exeter
Current Institution: University of Exeter
Date of Award: 2020
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Zeolitic imidazolate frameworks (ZIFs) are sub-family of metal organic frameworks with structures similar to traditional aluminosilicate zeolities. Consequently, ZIFs exhibit zeolite-type topologies with crystal structures, ultrahigh surface area, and excellent chemical and thermal stability, which makes ZIFs being an attractive candidate in various potential applications. Moreover, ZIF materials can act as outstanding templates or precursors to produce metal components on porous carbon nanocomposites, leading to a wide range of applications in energy storage and electrochemical utilisations. On the other hand, porous graphene could effectively avoid the stacking of graphene sheets, generating materials with high surface areas. Porous graphene can not only offer large aspect ratios which enhances the stability of porous frameworks to prevent collapse, but also provide the possibilities of multiple interactions with various species, both at the surface and through their porous frameworks, benefiting a rapid transportation of ions/molecules or charge carriers through the porous channels. In this thesis, the synthesis of ZIFs and graphene oxide (GO) derived nanocomposites were demonstrated and fully characterised. Moreover, the renewable-energy-related applications of these functional nanostructured derivatives were also evaluated and analysed. In brief, the main findings are as follows: 1. Developed a facile approach to produce highly efficient graphene-based cobalt sulfide and porous carbon composites, converted from one-step in-situ synthesised GO/ZIF-67 composites via sulfurisation and carbonisation at high temperatures. Different characterisation techniques have confirmed the CoS nanoparticles were homogeneously dispersed in carbon matrix. Moreover, the obtained nanocomposites exhibit much improved electrochemistry performance comparing with the reference material without graphene, and the electrocatalytic activities of the composites can be tuned by adjusting graphene content in the composites. 2. Apart from single metal sulfide, explorative research work were also performed to understand the potential of bi-metallic ZIF-67 derived nanocomposites. Homogenously dispersed nickel promoted cobalt sulfide/N, S co-doped carbon/graphene and iron promoted cobalt sulfide/N, S co-doped carbon/graphene have been successfully prepared via sulfurisation and carbonisation from Ni-substituted GO/ZIF-67 and Fe-substituted GO/ZIF-67, respectively. Due to the joint effect of graphene, N, S co-doped porous carbon and abundant metal-N moieties, the obtained nanocomposites exhibit not only remarkable OER catalytic activities with lowest onset/over potential, but also excellent HER activities with high current density and low onset potential, making them potential bifunctional electrocatalyst in water splitting. 3. Moreover, the derivatives of bi-metallic Fe-substituted GO/ZIF-67 have been further investigated. Iron promoted cobalt based nanoparticles homogeneously embedded in N-doped porous carbon and graphene via a facile one-step carbonisation of the in-situ as-synthesised composite. The obtained nanocomposites exhibit excellent electrochemical activities, which makes them promising electrode materials for catalysis and energy applications, owing to the increased surface area, hierarchical porous graphene and carbon structure, and bi-metal anchoring effect. Moreover, iron promoted cobalt oxide nanoparticles embedded in N-doped graphene and porous carbon by an efficient two-step carbonisation and oxidation of Fe-substituted GO/ZIF-67 has also been successfully developed. Due to the triple synergistic effect between iron oxide, cobalt oxides and N-doped porous graphene and carbon, the as-synthesised nanocomposites exhibit remarkable bifunctional activities towards both OER and HER in water splitting. 4. In addition, in all the studied mono- or bi-metallic component system, the effect of graphene oxide content as well as the sulfurisation/ carbonisation temperature have been well explored and optimised. It was found that the resultant nanocomposite sulfurised and carbonised at 800 °C, exhibited promising high-efficient catalytic activities. Meanwhile, owing to the introduction of a certain amount of graphene providing an increased electrical conductivity and more catalytic active sites, the optimum 5 wt% graphene contained nanocomposite shows the most remarkable electrochemical performance within the studied range of graphene content (up to 10 wt%).
Supervisor: Xia, Y. ; Zhu, Y. Sponsor: Engineering and Physical Sciences Research Council (EPSRC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available