Browsing by Author "Conrad, Olaf"

Now showing 1 - 6 of 6
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    Development of a semi–empirical reaction kinetic model for PEM fuel cells
    (2013) Fortuin, Adrian Charles; Conrad, Olaf; Levecque, Pieter B J
    In the drive to more sustainable energy production, polymer electrolyte fuel cells (PEFC) have been at the pinnacle of global research. One of the major drawbacks of PEFCs is therequirement for expensive noble metal catalysts (platinum and ruthenium). Furthermore 75% of the overpotential losses at the cathode are due to the activation of the oxygen reduction reaction (ORR). To reduce the platinum content requirements and understand the cause of the large overpotential of the ORR, a fundamental understanding of the reaction mechanism and the manner in which it proceeds under different operatingconditions is required. Presently, there still remains a large debate in literature around the mechanism followed by the ORR.This study developed a kinetic model from conventional kinetic isotherms and it is proposed that an associative adsorption mechanism occurs at a low overpotential resulting in the dissociation of the hydroperoxyl species determining the rate of the ORR at the cathode of the PEFC. In order to explain the above phenomena a kinetic model was developed, based on the Eley-Rideal mechanism. Furthermore, experiments were conducted at different oxygen partial pressures and low potentials whereby the associative mechanism is believed to dominate. Under these conditions linear sweep voltammograms were recorded. Regression of the derived kinetic model, by using the values for oxygen partial pressure, applied overpotential and kinetic current allowed for the determination of the kinetic constant of a polycrystalline platinum catalyst for ORR.
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    An investigation into the minimum dimensionality required for accurate simulation of proton exchange membrane fuel cells by the comparison between 1- and 3-dimension models.
    (2013) Shekhar, Karthik; Rawatlal, Randhir; Conrad, Olaf
    Hydrogen has been studied intensively as a potential energy carrier as it allows for a reduced carbon footprint in the environment. Fuel cell (FC) technology has been studied in detail to implement hydrogen as well as other renewable sources as a feasible fuel. Further development in fuel cell design is hampered by the lack of fundamental models which reveal the physical and chemical interactions. While computational fluid dynamics simulations are available, the timeframe for solving these simulations renders them unfeasible in any rigorous FC design optimisation. The objective of the present investigation was to determine the minimum dimension of a mathematical model that can accurately simulate processes occurring within a proton exchange membrane fuel cell (PEMFC). To this end, 1-D (directional axis perpendicular to the membrane) and 3-D steady state isothermal mathematical models were developed and simulated in order to investigate the transport of reactant species through the various layers of the cell at the anode side.
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    Oxygen reduction reaction on carbon supported dispersed platinum nanoparticles and extended platinum surfaces
    (2013) Taylor, Susan M; Conrad, Olaf; Levecque, Pieter B J
    To date, the cost of high platinum loadings in polymer electrolyte fuel cells (PEFCs) remains one of the main deterrents preventing their broad commercialisation. The reaction of interest in this work is the oxygen reduction reaction (ORR) occurring at the cathode side of the PEFC. The ORR has been studied at great length owing to the sluggish kinetics of this reaction and thus the necessity of the higher platinum loadings required on the cathode side of the PEFC. Platinum particle size and surface morphology are thought to be directly related to the catalytic activity of platinum towards the ORR (Mayrhofer et al., 2005) A better understanding of the effects of platinum particle size and surface morphology on the mechanism and kinetics of the ORR is critical if platinum loadings are to be reduced whilst maintaining the US Department of Energy (DoE) target performance levels (Schwanitz et al., 2012). This study focuses on the effects of platinum surface morphology on ORR activity by the development of platinum supported carbon electrodes. The surface morphology was modified by varying the platinum loading, such that the surface was taken from isolated nanoparticles to an extended surface. The effects of the changes in surface morphology and particle size on the ORR were then investigated. First a model electrode system was developed by magnetron sputter deposition of platinum onto a carbon black (VulcanXC72) surface supported on a glassy carbon electrode. The model system was then translated into a practical system, whereby platinum supported on carbon catalysts were prepared by metal-organic chemical vapour deposition (MOCVD) with increasing weight percentages of platinum. Similar trends were observed for the MOCVD catalyst system and the model electrode system. The trends of particular interest were the effects of platinum loading on hydrogen peroxide formation during the ORR and the double voltammeric CO oxidation peak observed with increased platinum loading during CO stripping voltammetry experiments. Hydrogen peroxide formation was observed at potentials below 0.8 V vs. SHE/RHE and it was found that hydrogen peroxide formation was predominant on dispersed platinum nanoparticles compared with platinum agglomerates. This is most likely related to peroxide species, a partial reduction product from oxygen to water, being able to desorb from the active surface without being further reduced to water on an adjacent site as the inter-particle distance is greater for the low loading samples relative to the higher loading samples in both systems. A clear indication of differing surface morphology between the low and high loading samples was observed by the double voltammeric peak detected during CO stripping voltammetry experiments. The low potential peak attributed to platinum agglomerates was seen to increase in amplitude as platinum loading increased. The corresponding high potential peak attributed to isolated nanoparticles, decreased in amplitude with increasing platinum loading. This observation alludes to an increase in particle agglomeration with platinum loading, and physical characterisation methods such as TEM and XRD showed an increase in the average particle size with increasing platinum loading, which supports the CO stripping voltammetry findings. The findings indicate a profound dependence of the ORR on platinum particle size and surface morphology. A better insight into these properties could assist in improved catalyst design and the enhancement of platinum utilisation and ORR activity of platinum.
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    Preparation and characterisation of Pt-Ru/C catalysts for direct methanol fuel cells
    (2014) Jackson, Colleen; Conrad, Olaf; Levecque, Pieter B J
    The direct methanol fuel cell (DMFC) is identified as a promising fuel cell for portable and micro fuel cell applications. One of the major benefits is that methanol is an energy dense, inexpensively manufactured, easily stored and transported, liquid fuel (Hamann et al., 2007). However, the DMFC's current efficiency and power density is much lower than theoretically possible. This inefficiency is predominantly due to the crossover of methanol from the anode to the cathode, Ru dissolution and Ru crossover from the anode to the cathode. In addition, the DMFC has a high manufacturing cost due to expensive catalyst costs and other materials. Catalyst expenses are further increased by catalyst loading due to low activity at the anode of the DMFC (Zhang, 2008). Hence, with increasing activity and stability of the Pt-Ru/C catalyst, catalyst expenditure will decrease due to a decrease in catalyst loading. In addition, performance will increase due to a reduction in ruthenium dissolution and crossover. Therefore, increasing the activity and stability of the Pt-Ru/C catalyst is paramount to improving the current DMFC performance and viability as an alternative energy conversion device. Pt-Ru/C catalyst synthesis method, precursors, reduction time and temperature play a role in the activity for methanol electro-oxidation and stability since these conditions affect structure, morphology and dispersivity of the catalyst (Wang et al., 2005). Metal organic chemical deposition methods have shown promise in improving performance of electro-catalysts (Garcia & Goto, 2003). However, it is necessary to optimise deposition conditions such as deposition time and temperature for Pt(acac)₂ and Ru(acac)₃ precursors. This study focuses on a methodical approach to optimizing the chemical deposition synthesis method for Pt-Ru/C produced from Pt(acac)₂ and Ru(acac)₃ precursors. Organo-metallic chemical vapour deposition (OMCVD) involved the precursor's vapourisation before deposition and a newly developed method which involved the precursors melting before deposition. An investigation was conducted on the effects of precursor's phase before deposition. The second investigation was that of the furnace operating temperature, followed by an exploration of the furnace operating time influence on methanol electro-oxidation, CO tolerance and catalyst stability. Lastly, the exploration of the Pt:Ru metal ratio influence was completed. It was found that the catalyst produced via the liquid phase precursor displayed traits of a high oxide content. This led to an increased activity for methanol electro-oxidation, CO tolerance and catalyst stability despite the OMCVD catalyst producing smaller particles with a higher electrochemically active surface area (ECSA).
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    Preparation of catalyst coated membranes using screen printing
    (2013) Hill, Matthew Raymond; Conrad, Olaf
    Of the various types of fuel cells, Polymer Electrolyte Fuel Cells (PEFCs) have already been demonstrated in transportation appliances from light-duty vehicles to buses and in portable appliances including laptops and cell phones. A key component of a PEFC is its platinum electrocatalyst. With an estimated 75% of the world’s platinum reserves and resources in South Africa, local development of this technology will allow South Africa to become a major player in the growing hydrogen economy. This project therefore forms part of the Department of Science and Technologies strategy, to develop fuel cell technology in South Africa. More specifically, this study aims to contribute to the development of membrane electrode assembly (MEA) platform technology at the HySA/Catalysis Centre. In order to achieve this goal, a catalyst coated membrane (CCM) fabrication procedure was implemented using a newly acquired screen printer. In this procedure, catalyst ink is forced through a mesh onto a substrate, where it can then be transferred to a membrane via decal transfer to form a CCM. Two gas diffusions layers can then be placed on either side of the CCM forming a 5-layered MEA. Characterisation techniques of the catalyst ink, CCM and 5-layered MEA were successfully implemented such that future researchers can expand on the ideas. Catalyst inks with varying amounts of isopropanol, 1,2-propanediol and water were screened for their suitability for screen printing. In particular the catalyst ink rheology required for a smooth and even printed surface was determined for a given screen and squeegee combination. With all the established steps in pace, screen printing proved to be a fast and reliable approach for CCM fabrication with potential for future scale up and commercialisation. The fabricated CCMs performed on a par with a commercial Ion Power CCM, but under performed in comparison to a commercial Johnson Matthey (JM) MEA. Possible reasons for this include improved materials in the JM MEA and cell conditions favouring the JM MEA. Future projects which specifically arise from this work entail an investigation into the water management of the fuel cell environment at HySA/Catalysis, as well as a modification of the various steps in order to optimise the process and in doing so manufacture commercially viable MEAs.
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    Towards reliable correlation of microporous layer physical characteristics and PEMFC electrochemical performance
    (2014) Crymble, Gregory A; Conrad, Olaf; Hussain, Nabeel
    Polymer electrolyte membrane (PEM) fuel cells are promising clean energy alternatives to non - sustainable fossil fuels. During fuel cell operation, external humidification of reactant gases is typically required in order to increase PEM conductivity for improved performance. However, the use of external humidification is costly and increases system complexity. Recently it has been found that by including a cathode microporous layer (MPL) in the membrane electrode assembly (MEA) , performance under dry conditions (no external humidification) can be significantly improved . However, the precise function of the MPL is not well understood and therefore there is little theoretical basis to optimisation of physical properties. One possible reason for this lack of understanding is the absence of a well-established fabrication, characterization and electrochemical testing methodology for MPL research. In particular, current research places little emphasis on the effect of MEA variance on the uncertainty in MPL electrochemical performance results. In this study a methodology is developed for fabricating, characterizing and testing MPLs to accurately correlate physical properties with in-situ electrochemical performance. MPLs of two significantly different thicknesses (approximately 20 and 50 μm in the thickest regions) were fabricated in - house using a doctor blade method and varying the ink composition. The pore structure and thickness of MPLs were characterized by mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM) and X-ray micro computed tomography (μCT).