Browsing by Author "Mohamed, Rhiyaad"

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    Electrocatalysis of oxide-based materials for the oxygen reduction and evolution reactions
    (2016) Mohamed, Rhiyaad; Levecque, Pieter B J; Fabbri, Emiliana
    Electrochemical devices, such as fuel cells and electrolysers, are said to be at the forefront of a renewable energy technology revolution centred on hydrogen as an energy carrier. These devices rely on the chemical reactions of oxygen, namely the oxidation of water to evolve oxygen (oxygen evolution reaction, OER) and hydrogen , carried out in electrolyser applications or the reverse reaction, the reduction of oxygen to water (oxygen reduction reaction, ORR) producing electricity in the case of fuel cells . Th e reactions of oxygen are however still hindered by extremely slow reaction kinetics. The resultant low efficiencies and associated high cost of electrocatalysts required hinder the widespread commercial success of these devices. In addition, current state - of - the - art electrocatalyst technologies suffer from severe corrosion during operation, presenting an additional barrier to commercialisation and ultimately delaying the successful implementation of a sustainable hydrogen economy. One primary goal of electrocatalysis research is thus the rational design of new materials with higher efficiencies. The fundamental understanding of the behaviour of the electrocatalyst materials towards these reactions will enable greater strides to be achieved in this area. To date much research has been conducted towards this end, however further progress is still required. This thesis details work towards the understanding of a new generation of electrocatalyst technologies for the OER and ORR. This study particularly explore s the use metal oxide based electrocatalyst materials for the oxygen evolution and reduction r eactions as employed in electrolyser and fuel cell applications respectively. The thesis is divided in two parts focusing individually on the OER and ORR respectively. New theoretical and experimental insight into the understanding of oxide electrocataly sts for the OER are discussed in Part I. Part II explores the ORR by studying metal oxides as both catalysts and catalyst support materials in alkaline and acidic environments respectively. Here the emphasis is placed on activity and durability of oxide ma terials under fuel cell operating conditions. The study confirms the promise of oxide based materials and highlights some of the challenges still present in their development for fuel cell applications. The final chapter presents a summary of the thesis. This study provides important insight and contributes towards the further understanding of the use of metal oxides for the OER and ORR. From this study several interesting and promising results were also obtained which warrant further intensive research and investigation. Directions for future research are discussed. [Please note: the full text of this thesis has been deferred until January 2018]
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    Fabrication of Catalyst Coated Membranes by Ultrasonic Spray for Proton Exchange Membrane Water Electrolysers
    (2023) Mawungwe, Nyasha; Mohamed, Rhiyaad
    Renewable hydrogen, referred to as green hydrogen (GH), holds significant importance in the endeavour to decarbonise the transportation and industrial sectors. GH is generated via the electrochemical process of water splitting, utilising excess renewable energy sources such as solar and wind, thus serving as a sustainable means of energy storage. The production of GH can be done in a proton exchange membrane water electrolyser (PEMWE), by splitting water into hydrogen and oxygen utilising an important component called the catalyst-coated membrane (CCM). The CCM is composed of a membrane coated with noble metal-based catalyst nanoparticles that make up the anode and cathode electrodes. The sluggish anode kinetics and the elevated cost associated with the CCM have acted as barriers to the widespread acceptance of PEMWEs. In this study, we used ultrasonic spraying for catalyst coating. Previous research suggests that optimising these parameters can enhance PEMWE performance and commercial viability. The research conducted involved an ultrasonic spraying parameter variation and an anode catalyst loading study. The ultrasonic spraying variation investigated the nozzle height and nozzle speed. The anode catalyst ink was formulated from a commercial catalyst and applied to each membrane forming a half CCM, and thereafter, combined with a commercially developed cathode to form a full CCM. The CCMs were physically characterised, and electrochemically tested. The results were compared to assess the impact of ultrasonic spraying parameters and anode loading on performance and catalyst utilisation. The fabricated samples with approximately 2 mg Ir anode loading were further compared to a commercial CCM benchmark, considering the CL surface, microstructure, performance, and catalyst utilisation. The results showed the influence of spraying parameters, catalyst type, and loading on microstructure, performance, and utilisation. This showed the importance of optimising parameters and loading to develop comparable low-loaded catalyst layers to assist PEMWE adoption.
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    Iridium oxide supported on antimony-doped tin oxide as an electrocatalyst for the oxygen evolution reaction
    (University of Cape Town, 2020) Rajan, Ziba Shabir Hussein Somjee; Mohamed, Rhiyaad; Binninger, Tobias
    The generation of high purity hydrogen by renewable, sustainable means is a crucial building block towards the realisation of a carbon-free energy economy. Proton exchange membrane water electrolysis (PEMWE) offers a promising route for the generation of clean hydrogen, using renewable energy, for both stationary and mobile energy storage applications, and as a feedstock for the chemical industry. As water electrolysis is an electrochemical redox reaction, cathodic hydrogen evolution cannot occur without an efficient, and rapid anodic oxygen evolution reaction (OER). While both iridium and ruthenium oxides are state-of-the-art OER catalysts in acidic environment, the latter undergoes dissolution under anodic OER conditions much more rapidly than the former, and this makes iridium oxide the most suitable catalytic material for electrolyser anodes. Several strategies have been explored as a means to lower the iridium content in OER catalysts, and of these, the use of cheap, stable support materials has been seen as a promising means to produce highly active, durable catalysts, by enhancement of the electrocatalytically active surface area. In this thesis, the viability of an organometallic chemical deposition method for the deposition of IrOₓ nanoparticles on antimony-doped tin oxide (ATO) support is investigated. The effect of the gas environment (oxygen or argon) and the temperature used for the deposition was examined. The ex-situ OER performance of the synthesised electrocatalysts was evaluated using the rotating disk electrode technique. Using X-ray photoelectron spectroscopy (XPS) and high-resolution transmission scanning electron microscopy (HR-STEM), the physical properties of the synthesised IrOₓ/ATO catalysts were elucidated, in order to understand the observed oxygen evolution activity and stability of IrOₓ/ATO in relation to the OMCD technique. In addition to developing an understanding towards the physical and electrochemical properties of the synthesised materials, strategies to optimise the Ir yield achieved by the organometallic chemical deposition process were explored.