Browsing by Author "Hussain, Nabeel"

Now showing 1 - 4 of 4
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    Open Access
    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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    Open Access
    Microchannel flow fields for polymer electrolyte fuel cells
    (2015) Chivengwa, Tapiwa; Hussain, Nabeel
    Fuel cell technology represents an efficient and relatively quiet way of generating electricity. Among the various types of fuel cells, the polymer electrolyte fuel cell (PEFC) is the leading candidate for portable, automotive and more recently stationary applications. One of the key challenges affecting both the performance and durability of low temperature PEFCs is water management. Various water management strategies in PEFCs have been employed to date ranging from manipulation of operating conditions, fuel cell component design and flow field design to name a few. The optimisation of the flow field design for water removal has primarily focused on the use of flow channels which are in the minichannel range. This study investigated the use of a microchannel flow field design (channel hydraulic diameters of less than or equal to 200 ìm) for a low temperature PEFC. Specifically it focused on the effect of using a microchannel design on overall fuel cell performance, pressure drop and the cell voltage behaviour over time. In addition the effect of different operating conditions was also investigated. The overall aim was to develop a more comprehensive understanding of the use of a microchannel based flow field system with specific focus on water management. Fuel cell testing of two different flow field designs, namely a microchannel design and a more conventional commercial minichannel design, was performed in a single cell set up. Two operating conditions, cathode flow rate and cell compression, were varied and the effect on overall fuel cell performance and limiting current was investigated. Several diagnostic measurements including polarization curve, high frequency resistance, electrochemical impedance spectroscopy, pressure drop co-efficient and cell voltage monitoring were conducted to understand the water management behaviour and trends in the two different aforementioned flow field designs.
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    Open Access
    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).
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    Water management strategies for polymer electrolyte fuel cells (PEFCs) employing microchannel flowfields
    (2015) Daniels, Simone Monique; Hussain, Nabeel; Tanaka, Shiro; Schwanitz, Bernhard
    Polymer electrolyte fuel cells (PEFCs) represent a promising energy conversion technology for automotive and portable applications. In order to achieve the high power densities required for these applications, the fuel cell needs to be operated in the high current density region where the rate of water production is at a maximum. This typically leads to the build-up of liquid water in the porous media and flowfield compartments of the fuel cell. The build-up of liquid water inhibits reactant gas transport to the catalyst layer, leading to a phenomenon called flooding. Flooding causes a rapid drop in cell voltage and is detrimental to fuel cell performance and durability. Microchannel flowfield designs possess characteristics which could potentially improve water removal from the fuel cell and also reduce the fuel cell system complexity. There is limited knowledge on the use of microchannels flow field designs in PEFCs, specifically how different operating conditions and different membrane electrode assembly (MEA) designs affect the overall performance and water management of a fuel cell using microchannel flow fields. This study investigated two water management strategies for PEFCs employing microchannel flowfields, namely manipulation of operating conditions and modification to the design of components within the MEA. Four different gas diffusion layer (GDL) cases were tested in a single cell environment at four different cathode flowrates and stoichiometric ratios. The cases consisted of a carbon GDL and three variants of a uniform structured metal GDL. The three metal GDL designs varied in terms of the wettability of the microporous layer coated on the surface of the metal GDL. Several in-situ diagnostic tests, namely polarisation curves, electrochemical impedance spectroscopy (EIS), pressure drop and voltage stability tests were conducted to determine the overall fuel cell performance and water management characteristics of the different GDL cases.