Browsing by Author "Levecque, Pieter B J"

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    Open Access
    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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    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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    Evaluation of metal nitrides and borides as alternative electrocatalyst support materials for polymer electrolyte fuel cells
    (2014) Khoza, Thulile P; Blair, Sharon; Levecque, Pieter B J
    Polymer electrolyte fuel cells (PEFCs) have wide variety of commercial applications, however due to poor durability and high cost, this technology has currently not reached its commercialization stage. Poor durability is mainly attributed to carbon support corrosion during start-up and shut-down of the fuel cell. Corrosion of the electrocatalyst support materials has numerous adverse effects on the performance of the fuel cell, such as weakening of metal-support interaction which results in Pt detachment, dissolution and sintering. Hence, the electrochemical active surface area is significantly reduced. It is clear that there is an urgent need for more robust, high performance alternative support materials to carbon. In this study, metal nitrides and borides (TiN, ZrN, TiB₂ , ZrB₂ and LaB₆) were evaluated as potential support materials, in an attempt to improve the durability and performance of PEFCs.
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    Formation of Pt-Based Alloy Nanoparticles Assisted by Molybdenum Hexacarbonyl
    (2021-07-14) Leteba, Gerard M; Mitchell, David R G; Levecque, Pieter B J; van Steen, Eric; Lang, Candace I
    We report on an optimized, scalable solution-phase synthetic procedure for the fabrication of fine-tuned monodisperse nanostructures (Pt(NiCo), PtNi and PtCo). The influence of different solute metal precursors and surfactants on the morphological evolution of homogeneous alloy nanoparticles (NPs) has been investigated. Molybdenum hexacarbonyl (Mo(CO)6) was used as the reductant. We demonstrate that this solution-based strategy results in uniform-sized NPs, the morphology of which can be manipulated by appropriate selection of surfactants and solute metal precursors. Co-surfactants (oleylamine, OAm, and hexadecylamine, HDA) enabled the development of a variety of high-index faceted NP morphologies with varying degrees of curvatures while pure OAm selectively produced octahedral NP morphologies. This Mo(CO)6-based synthetic protocol offers new avenues for the fabrication of multi-structured alloy NPs as high-performance electrocatalysts.
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    Influence of catalyst ink mixing procedures on catalyst layer properties and in-situ PEMFC performance
    (2016) Jacobs, Clayton Jeffrey; Levecque, Pieter B J; Hussain, Nabeel; Schwanitz, Bernhard W
    Despite the benefits of fuel cell technology its advancement to being commercially functional is hindered by a number of crucial factors. These factors are often associated with the lack of appropriate materials or manufacturing routes that would enable the cost of electricity per kWh to compete with existing technology. Whilst most research efforts have been directed towards developing more active catalysts, the amount of catalyst required in the fuel cell can be further reduced by improving the platinum utilisation in the membrane electrode assembly. The platinum utilisation is a strong function of the catalyst layer preparation step and there remains significant scope for optimisation of this step. Whereas significant work has been conducted into the different components of the catalyst ink there is limited work and understanding on the influence of the mixing method of the catalyst ink. This study will focus on the influence of the mixing technique on the catalyst ink properties and on the final fuel cell performance. Specifically, the study will investigate the effect of the three different mixing techniques on (i) catalyst ink quality (ii) the physical properties of the resultant catalyst layer and (iii) the in-situ electrochemical performance of the membrane electrode assembly. A large set of characterisation techniques were chosen to effectively study the step wise processing of the catalyst layer, and fuel cell performance. The results presented here include a comparison of the various mixing techniques and a comprehensive 2 x 2 factorial design into the individual techniques. The results suggest that high energy mixing is required for effective distribution of catalyst layer components, an even catalyst layer topography and a highly functional ionomer network which consequently, enhances performance. The mixing energy referred to involves prolonged mixing time, enhanced mixing intensity or a combination of the two. During bead milling of catalyst inks, high intensity mixing seems to be beneficial however, prolonged mixing time appears to be detrimental to the ionomer film structure. During high shear stirring and ultrasonic homogenisation of catalyst inks, the ink mixture significantly heats up. It has been observed that at higher temperatures, Nafion elongates and the contact with catalyst agglomerates is enhanced. High shear stirring of catalyst inks seems to be most effective at high agitation rates. High mixing energies result in high shear forces and in addition, high mixing temperatures which appear to be beneficial to establishing an effective catalyst/Nafion interface, enhancing the three phase boundary observed during in-situ testing. Ultrasonic homogenisation seems to be more effective at prolonged sonication times. Due to the erosive nature of ultrasonic dispersion, sufficient time is required to establish a well dispersed and distributed catalyst ink. However, the nature of particle size distribution resulting from ultrasonication shows that inks are unstable and is not recommended for high throughput processing. Overall, fuel cell performance is not significantly affected by the mixing step however; mixing does have an observable impact on catalyst layer formulation. Generally, when optimizing membrane electrode assembly fabrication, mixing parameters should be carefully chosen. This goes without saying that parameters need to be effectively studied before foregoing catalyst ink processing.
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    Influence of particle size and morphology of Pt₃Co/C on the oxygen reduction reaction
    (2015) Hlabangana, Ntandoyenkosi; Levecque, Pieter B J; Schwanitz, Bernhard W
    Polymer electrolyte fuel cells have shown great potential in providing clean energy with no emissions. The kinetics of the cathode reaction, i.e. the oxygen reduction reaction (ORR) are sluggish necessitating high loadings of the catalyst metal, i.e. platinum. Platinum is a limited resource and expensive. Its price has been one of the major drawbacks in wide scale commercialisation of fuel cells. In an effort to improve the activity of the catalyst and therefore reduce Pt loadings on the catalyst, Pt can be alloyed with transition metal elements (e.g. Ni, Co and Fe) to form bimetallic catalysts. Alloying has been known to improve the activity and stability of a catalyst for the ORR. The enhanced activity of the alloys originates from the modified electronic structures of the Pt in these alloy catalysts which reduces the adsorption of spectator species therefore increasing the number of active sites for the ORR (Wang et al., 2012 (2)). The aim of this study was to gain a better understanding of the influence of Pt alloy particle size and active surface morphology on the ORR activity. The Pt alloy that was investigated was Pt₃Co/C. The surface morphology was modified by varying the Pt/Pt₃Co loading on a carbon support. The catalysts were prepared using thermally induced chemical deposition. The support used was Vulcan-XC-72R. The effects of varying the metal loadings on the ORR was investigated. The loadings that were investigated were 20, 40, 60 and 80 wt. % Pt and Pt₃Co. The alloy catalysts were subjected to annealing at 900 °C and acid leaching. The catalysts were analysed using electrochemical characterisation techniques such as cyclic voltammetry, CO stripping voltammetry, rotating disk electrode and rotating ring disk electrode. Physical characterisation of the catalysts was also implemented. The techniques used were x-ray diffraction, thermogravimetric analysis and transmission electron microscopy. The Pt particles on the carbon support were found not to agglomerate significantly despite the loading being increased. This trend was also observed for the Pt₃Co/C catalysts even after heat treatment and leaching. The lack of agglomeration was credited to a new reactor system developed in this work. The particle growth increased from low loadings to high loadings for both the Pt/C and Pt₃Co/C catalysts. Particle growth was more significant for the Pt₃Co/C catalysts at high loadings. At lower loadings (20 and 40 wt. %) the particle sizes between the Pt/C and Pt₃Co/C catalysts were comparable despite the Pt₃Co/C catalysts undergoing annealing and leaching. The mass specific activity of the Pt/C catalysts was not improved by alloying with the exception of the 20 wt. % catalyst which saw an enhancement factor of 1.66. The surface specific activity of the Pt/C catalysts was improved significantly with factors of 2.40 and 3.11 being recorded for the 20 and 80 wt. % Pt₃Co/C catalysts respectively. The enhancement factors of the intermediate loadings (40 and 60 wt. %) were lower and fairly similar at 1.30 and 1.35 respectively.
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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 inorganic nanostructured support materials for polymer electrolyte fuel cells
    (2015) September, Caelin Gee; Levecque, Pieter B J; Schwanitz, Bernhard W
    Polymer electrolyte fuel cells (PEFCs) have been identified as a safe, clean and reliable alternative energy conversion technology to conventional, fossil fuel based, ones. However, the hindrance to worldwide commercialisation of this technology lies in the poor durability and high costs associated with the current carbon supported platinum (Pt/C) catalysts. Carbon support corrosion and Pt dissolution/aggregation on the catalyst layer within the fuel cell have been confirmed as the major contributors to the degradation of the Pt/C (Shao, et al., 2007). Attention needs to be paid to the improvement of catalyst components to produce an electrocatalyst with better degradation resistance and low Pt loading in order to overcome these two major commercialisation barriers. The physico-chemical and electronic interaction between the Pt catalyst and the support material play a crucial role in the catalytic activity and stability of the electrocatalysts (Wang, et al., 2011). A comprehensive understanding of the effects of catalyst support material and morphology on the mechanism and kinetics of the oxygen reduction reaction (ORR) needs to be developed. This study investigated alternative, novel catalyst support materials and structures for the catalyst layer as opposed to carbon for PEFC applications. This material consisted of TiB2 electrospun nanofibers, powder and crushed electrospun nanofibers. Methods used to reliably and accurately deposit Pt onto these materials were identified, developed and analysed. These methods include platinum deposited onto TiB2 powder, electrospun crushed nanofibers and nanofiber mats via DC magnetron sputter deposition and thermally induced chemical deposition (TICD). The synthesised catalysts were physically characterised using X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Inductively Coupled Plasma Optical emission spectrometry (ICP-OES). Platinum effectively deposited on the TiB2 support structures via these deposition techniques within two standard deviations of the desired Pt loadings.
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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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    SiC and B₄C as electrocatalyst support materials for low temperature fuel cells
    (2017) Jackson, Colleen; Levecque, Pieter B J; Kramer, Denis; Russell, Andrea E
    Supported nano-catalyst technologies are key for increasing the catalyst utilisation and achieving economically feasible platinum metal loadings in hydrogen polymer electrolyte fuel cells (PEFCs). High surface area carbons are commonly utilised as support materials for platinum due to low cost, large surface areas and high conductivity. However, PEFCs using this technology undergo oxidation of carbon supports, significantly reducing the lifetime of the fuel cell. In this work, silicon carbide and boron carbide are investigated as alternative catalyst support materials to carbon, for the oxygen reduction reaction for low temperature fuel cells. Electrochemical testing, accelerated degradation studies as well as advanced characterisation techniques were used to clarify the structure-property relationships between catalyst morphology, metal-support interaction, ORR activity and surface adsorption onto the Pt nanoparticles. Extended X-ray Absorption Fine Structure (EXAFS) analysis gave insights into the shape of the clustered nanoparticles while X-ray Photoelectron Spectroscopy (XPS) and in-situ X-ray Absorption Near-Edge Spectroscopy (XANES) analysis provided information into how the metal-support interaction influences surface adsorption of intermediate species. Electronic metal-support interactions between platinum and the carbide supports were observed which influenced the electrochemical characteristics of the catalyst, in some cases increasing the oxygen reduction reaction activity, hydrogen oxidation reaction activity and Pt stability on the surface of the support.
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    Synthesis, characterization and catalytic investigations of Pt-based binary (bimetallic) and ternary (trimetallic) nanoparticles
    (2016) Leteba, Gerard Malefane; Lang, Candace; Levecque, Pieter B J
    This work tests the hypothesis that nanoparticles of 75 at.% platinum (Pt) composition and anisotropic morphology, will outperform standard catalysts in (PEMFC) hydrogen fuel cells. A survey of the scientific literature on this topic is first presented. The synthetic strategies which were developed for the preparation of novel Pt-based binary (bimetallic) and ternary (trimetallic) nanoparticles, containing nickel (Ni), cobalt (Co) and/or vanadium (V), are then described. The synthesis protocols for solution-grown colloidal nanoparticles all required the heat-up of a chemical mixture (of metal precursors, surfactants as stabilizers, solvents and/or reductants) from room temperature to high temperatures (up to 310 °C), for thermal decomposition or thermal co-reduction. These protocols were successful in producing nanostructures of high quality, with exceptional solubility in polar solvents such as chloroform after repeated washing and drying. Detailed microstructural investigations of the synthesized nanoparticles were carried out using scanning transmission electron microscopy (STEM), TEM and X-ray diffraction (XRD). The nanoparticles were anisotropic with composition around 75 at.% Pt. Depending on the particular synthesis protocol, the as-prepared nanoparticles exhibited different morphologies, surface facets, size and structure (alloy or core-shell). To measure the oxygen reduction reaction (ORR) functionality of these nanoparticles, electrochemical measurements were conducted, including cyclic voltammetry (CV), carbon monoxide stripping voltammetry (CO-stripping) and rotating disk electrode measurements (RDE). These measurements determined (a) electrochemical surface area, (b) mass-specific activity and (c) area-specific activity; which were used to compare the performance of the synthesized nanoparticles with the performance of a standard catalyst. The synthesised nanoparticles, containing 75 at.% Pt and having anisotropic morphologies, exhibited better catalytic functionality than the standard catalysts currently in widespread use. The enhanced functionality of these alloy nanostructures is attributed to their anisotropic nature and structure (mixed or core-shell). It is shown accordingly that high surface area nanoparticles, with platinum composition around 75 at.%, are more effective than the best catalysts currently in use. Subsequently, electrochemical measurements were used to determine longevity: catalytic functionality was measured after cycling for considerably longer than the norm in nanoparticle research (5000 cycles). These measurements show a decay in catalytic activity after prolonged potential cycles, although the final value is similar to the initial value for commercial Pt catalyst. This decay is suggestive of alloying dissolution and surface facet deformation; further work is recommended.