Zeolitic Imidazolate Frameworks (ZIFs) Derived Metal Sulphides/Carbon Nanocomposites: Synthesis and Electrochemical Applications

Abstract

In the past decade, Zeolitic imidazolate frameworks (ZIFs) as precursors towards functional materials have attracted enormous attention. The easy modification of the ZIF structure itself, as well as the easy incorporation of various materials by different manners to form ZIF-based composites, have resulted in a great kingdom of functionalised ZIFs. Moreover, functional ZIFs derived nanocomposites of desired compositions and sophisticated structures can be obtained under suitable pyrolysis conditions and show great potential in various catalytic applications. Built on a pertinent literature review with special emphasis on the application in electrocatalytic water splitting, this thesis focuses on the rational design, synthesis, characterisation and electrochemical evaluation of ZIF-based composites and their nanocomposite derivatives. First, a facile and cost-effective one-pot in situ encapsulation method was applied for the synthesis of phosphotungstic acid (PTA) encapsulated ZIF-67, and the bimetallic WS2/Co1-xS/N, S co-doped porous carbon nanocomposites were derived from one-step sulphurization/carbonisation of the PTA@ZIF-67 composites. Significant improvement in electrocatalytic activity towards both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) was observed. Then, a different post-synthetic strategy to introduce PTA into ZIF-67 is utilised. Moreover, phosphomolybdic acid (PMA), which is unable to be introduced into ZIF-67 with the previous in situ encapsulation method, has been successfully encapsulated into modified ZIF-67 with this new synthesis approach. The HER and OER active, defect-rich ultra-fine Co-Mo-S/N, S-doped porous carbon nanocomposites and WS2/Co1-xS/N, S co-doped porous carbon nanocomposites were derived from the modified ZIFs. This new synthesis strategy to modify acid-sensitive ZIFs with selected compounds offers an alternative approach to develop novel transition metal sulphide/carbon composites for various applications. Finally, the in situ encapsulation method is extended to a metal organic framework (MOF)-based composites (PMA@MIL-100), which demonstrates the universal applicability of the method. Bimetallic Fe-Mo sulphide/S-doped carbon nanocomposite was derived from the PMA@MIL-100 composite through chemical vapour sulphurization at high temperatures and showed a dramatic improvement in the HER performance.