Li, Juntao; (2021) Energy efficient strategy for designing metal–organic frameworks based bifunctional electrocatalysts and high performing rechargeable zinc-air batteries. Doctoral thesis (Ph.D), UCL (University College London).

Preview

Text PhD thesis_Juntao Li_Final.pdf Download (6MB) | Preview

Abstract

To fulfil the increasing demands for efficient and clean energy storage and conversion technologies, including fuel cells, water electrolysis and metal-air batteries are of great importance. These energy technologies are based on various electrochemical catalysis processes, such as hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Huge efforts have been dedicated for designing the efficient electrocatalysts. In spite of the benchmark materials from the precious metal compounds (Pt/C for ORR, IrO2 for OER), the most common routine for synthesising bifunctional catalysts is carbonising organic, inorganic and polymeric materials at high temperature and then postsynthesis chemical modification. However, production of such materials via pyrolysis poses significant challenges for controlling the composition, functionality, heterogeneity, porosity and nanophase (i.e., aggregation of metal centres) properties, as well as for mass production. Hence, in this thesis work a straightforward and energy efficient routes to produce highly active electrocatalysts are disclosed and discussed. In the first work, a new strategy to extract the electrocatalytic activity from as-received zeolitic imidazolate frameworks (ZIFs) is developed without the need for routinely performed postsynthesis chemical modification. This represents a step forward in experimental design and yields materials with near best in class performance. Metal-organic framework (MOF)-/ZIF-derivatives nanostructure via post-synthesis thermolysis have opened up phenomenal interest for electrocatalysis applications. Contrary to this, the first work discovers the interesting electrocatalytic activity of as-synthesized bimetallic ZIFs(Co/Zn) directly without carbonisation and establishes OER activity trend with the substitution of cobalt metal centres for zinc. An 4 enhanced activity and trend with respect to particle size of ZIF-67 is achieved, where identical structural parameters are maintained. Furthermore, a significantly enhanced OER activity, >50%, in ZIF-67/carbon black or ZIF-67/carbon paper conducting supports is obtained. A long term durability of ZIF-67/carbon paper during 24 hours continuous operation shows exceptionally improved activity and stability. In the second work, based on the comprehensive understanding for the readily synthesized MOF structure, a new composite of cobalt based ZIF and platinum carbon black (Pt/CB) in 1:1 mass ratio (ZIF-67@Pt/CB) is designed via facile solution mixing at room temperature, and without carrying out high temperature carbonization and/or calcination. This hybrid compound shows a promising bifunctional electrocatalytic activity for oxygen (OER and ORR), the key charge and discharge mechanisms in a battery. The as produced ZIF-67@Pt/CB exhibits a significantly enhanced catalytic stability compared to Pt/CB and delivers a high performance with excellent long term stability. Overall, the readily available ZIF-67 can be easily turned into a high performing zinc-air battery cathode material. This thesis work shows commercial feasibility of zinc-air batteries as ZIFs can be reproducibly synthesized in mass scale and applied as produced. The third work demonstrates a replicate synthetic route to highly active electrocatalyst from ZIF starting materials without the need for carbonisation or other post-synthesis modification. Inspired by the second work, in that the post-electrochemical studies reveal an electrochemically driven insitu development of OER active cobalt-(oxy)hydroxide nanophase. Therefore, as-synthesized ZIF67 is being in-situ electrochemically activated on the air cathode directly by coating thin film on the carbon paper gas diffusion layer current collector. The in-situ electrochemical activation has been carried out by performing charge-discharge cycling tests under various applied current densities as well as cyclic duration. In addition to this, ZIF-67 samples with various crystallite 5 sizes (particle size of polyhedrons between 50 nm and 2000 nm) have also been studied. These electrochemical and physical conditions help to understand the true activation mechanism of the ZIF-67 and further to fine tune the structure-activation-property relationships. The electrocatalyst generated via in-situ electrochemical activation under optimized particle size and activation conditions delivers a remarkable performance in the zinc-air batteries, outperforming the most of the carbon based as well as commercial benchmark standard Pt/CB+IrO2/CB bifunctional catalysts in the literature. The in-situ activated ZIF-67 air cathode catalyst based zinc-air batteries repeatedly deliver outstanding long cycling stability for over 1000 h (6600cycles) and maintaining high power density of ~180 mW cm-2 , whereas the reference Pt/Ir-based catalyst air cathode based cell dies after 50 h (200 cycles) with a rapid degradation in the power density. Therefore, given the urgent and rapidly growing global interest in energy conversion and storage materials, and the key material and conceptual advances presented, this thesis work will guide for further advancements and enable identification of electrocatalysts from as-produced ZIFs alone, without resorting to energy-intensive post-processing methods.

Download activity - last month

Export as JSONCSVXML

Download activity - last 12 months

Export as JSONCSVXML

Downloads by country - last 12 months

Export as CSVXMLJSON

Archive Staff Only

View Item