Fabrication and characterisation of nanostructured Au for improved electrocatalytic performance in ethanol oxidation reaction

• Main Author: Cao, Xun
• Other Authors: Huang Yizhong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Materials::Nanostructured materials; Engineering::Materials::Material testing and characterization
• Online Access: https://hdl.handle.net/10356/143810
• Institution: Nanyang Technological University

In the modern society, fossil fuels are absolutely the major energy sources in daily lives. However, the over-usage of fossil fuels not only leads to quick depletion of their reserves, but also causes environmental pollution when they are burnt. As such, apart from degradation of pollutants like hazardous chemicals and greenhouse gases, there are two key parts to counteract the current energy crisis – sourcing for alternative and renewable clean energies, as well as to develop energy storage and energy conversion systems. Although different sources of renewable energy are playing their individual role in the energy supply, electrochemical energy will be the dominant one in a long term energy scenario. A general requirement for electrochemical energy conversion and storage is to develop efficient and cost-competitive devices, such as batteries, supercapacitors and fuel cells. Ethanol, being a potential substituent fuel, has wide availability, cheap price, low toxicity and high energy density, therefore direct ethanol fuel cell (DEFC) has long been a popular research topic and ethanol oxidation reaction (EOR) is one of the key focuses. Electrochemical conversion and storage at electrodes undergo complex processes, but most of them are related to electrocatalysis process. Furthermore, electrocatalysis process is one of the best solutions to environmental pollution, as it degrades many hazardous materials into less harmful ones without too much outsourced energy supply. Therefore, development of novel electrocatalysts (electro-active materials) with low cost and high efficiency is an essential step in the advancement of pollutant degradation and next-generation electrochemical energy storage/conversion systems. In this project, an important concept established is that there are two distinct ways to improve the electrocatalytic performance of Au in EOR, which are increasing the specific surface area and raising the surface energy. Nanostructured Au electrocatalyst was fabricated in two forms – thin films and nanoparticles (NPs). Micro-/nanostructured arrays were fabricated on Au thin films by two lithographical methods, one was a combination of nanosphere lithography (NSL) and reactive ion etching (RIE) and the other was nanoimprint lithography. Through optimisation of parameters, the ordered Au array structures could have almost 100% effective coverage area over the entire substrate. Transmission electron microscopy (TEM) characterisation achieved direct observation of atomic-scale active sites and partial lattice rotation, which greatly contributed to the improvement of electrocatalytic performance of Au. On the other hand, Au NPs were fabricated via a non-traditional way. Under the ion bombardment in a precision ion polishing system (PIPS), homogeneous and ultra-small Au NPs were found to have single crystallinity; with a suitable set of parameters, the NPs rendered crystal twinning effect, which created localised stress centres that raised the surface energy of the NPs. Literatures have reported that EOR may proceed via two distinct pathways (C1 and C2) with totally different end products; C1 pathway is the more desired one that causes full oxidation of ethanol and produces CO2 as a result. In this project, the catalytic performance of Au in EOR was tested via electrochemical methods. Through cyclic voltammetry (CV) scans of the samples, Au rendered high efficiency in EOR with low energy input requirement. The CV scans were carried out under different physical parameters – such as having different pH values and ethanol concentrations in the electrolyte – to have a thorough understanding of the catalytic nature of Au in EOR. Results had shown that at extreme pH values, the electrolyte has high concentrations of mobile charge carriers that favour forward proceeding of EOR. The variation of ethanol concentrations quantified the detecting limit of ethanol, which was usually ~ 5.0 mM and with the presence of atomic-scale active site, this value could reach as low as 1.0 mM. Electrochemical impedance spectroscopy and chronoamperometry tests were also conducted to determine the double layer capacitance and the amount of active sites involved in the electrocatalysis, as well as the stability of the samples during the process. Results had demonstrated that the samples exhibited low charge transfer resistance and excellent double layer capacitance with large amount of participating active sites. Long continuous CV scans revealed that in alkaline medium, C1 pathway could become increasingly preferential over C2 pathway, and thus making nanostructured Au useful in the application of DEFC. Excellent controllability and repeatability, usage of low-hazard materials as well as cost-effective large area fabrication are the key features in this project, which creates opportunities for large-scale fabrication and industrial applications.