Browsing by Author "Chamier, Jessica"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
- ItemOpen AccessComparative analysis of Polymer Electrolyte Membrane (PEM) fuel cells(2018) Balogun, Emmanuel O; Barendse, Paul; Chamier, JessicaPer-Fluoro-Sulphonic-Acid (PFSA) ionomers have been singled out as the preferable ionomers for making the Polymer Electrolyte Membrane Fuel Cells (PEMFC) membranes owing to their extensive intrinsic chemical stability and super sulfonic acid strength which is core to the PEMFC proton conductivity. This thesis presents a deeper analysis into these PFSA ionomer membrane electrode assemblies (MEA), presenting an electrochemical-analytical comparative analysis of the two basic types, which are the Long-Side-Chain (LSC) Nafion® and the ShortSide-Chain (SSC) Aquivion® ionomer MEA with emphasis on performance and durability which are currently not well understood. In particular, electrochemical circuit models and semiempirical models were employed to enable distinguishable comparative analysis. Also, in this thesis, we present a further probe into the effect of ionomer ink making processes, critically investigating the effect of the High Share Dispersion (HSD) process on both the Nafion® and Aquivion® ionomer membrane electrode assembly (MEA). The findings in this research provides a valuable insight into the performance and durability of PFSA ionomer membrane under various application criteria. The effect of operating parameters and accelerated stress testing (AST) on the PFSA ionomers was determined using electrochemical impedance spectroscopy (EIS) and electronic circuit model (ECM) analysis. The result of this study, shows that the ionomer ink making process for Nafion® and Aquivion® MEAs are not transferrable. Analysis of the PEMFC performance upon application of the high shear dispersion (HSD) process showed that Nafion® MEA had a 10.47% increase in voltage while the Aquivion® MEA had a 2.53% decrease in voltage at current density of 1.14A/cm2 . Also, upon accelerated stress testing, the Nafion® showed a 10.49% increase in its voltage while the Aquivion® on the other hand had a 7.16% decrease in voltage at 0.66A/cm2 . Thus indicating the HSD process enhances the performance of the Nafion® MEA and inhibits the performance of the Aquivion® MEA.
- ItemOpen AccessIridium oxide supported on graphitized carbon for use as reversal tolerant anodes in pem fuel cells(2020) Labi,Tita N; Chamier, Jessica; Van Schalkwyk, FrançoisOne way in which potential reversal occurs in Proton Exchange Membrane Fuel Cells (PEMFCs) is when the H2 supply is insufficient to meet the load requirements of the cell. During cell voltage reversal, carbon oxidation and water electrolysis occur at the anode to maintain the supply of protons and electrons to the cathode. These reactions are nonspontaneous and consume energy from the membrane electrode assembly (MEA), causing irreversible damage through carbon corrosion. A material-based approach to prevent or reduce damage to the anode during cell reversal is to include an oxygen evolution reaction (OER) catalyst which will decrease the overpotential for water electrolysis. The addition of the OER catalyst to generate a reversal tolerant anode (RTA) will therefore decrease the occurrence of the carbon corrosion at cell potentials below -1.2 V. Due to the harsh conditions of fuel cell operation, the OER catalyst needs to provide both high activity and durability. Studies have shown that among the noble metal electrocatalysts, iridium (Ir) in its oxide form is among the highest for OER activity in acidic media. McCrory et al., (2013) also found that of all their tested metal oxide systems, IrOx was the most stable in acidic solutions. IrOx is an efficient catalyst for the OER but the use of pure IrOx is limited due to high costs and low surface area in its crystalline form. Several studies have therefore focused on supporting the IrOx on various materials to increase surface area and stability. Ideal catalyst supports used in electrochemical cells need to have a porous nanostructure and high electronic conductivity for electron transfer through the external circuit. Carbon remains the preferred commercial support due to its low costs, however, it undergoes corrosion under certain conditions during operation. Graphitized carbon supports have been gaining more attention due to their corrosion resistance and superior electrical conductivity. The aim of this study is therefore to use graphitized carbon supported iridium oxide to generate reversal tolerant anodes in an MEA. The IrOx supported graphitized carbon catalyst was synthesized using a microwave assisted polyol deposition technique. The supported catalyst was characterized using XRD, TEM, TGA, XPS and EDX analysis. The nanoparticles were well dispersed over the graphitised support and in the range of 4-10 nm. XPS analysis showed that the main oxidation states of Iridium were Ir3+ and Ir4+ which are proven to be the main states responsible for the OER. After comparison with commercial catalysts, it was found to have a good balance between activity and durability having an activity of 0.141 A/mg Ir towards the OER. The supported IrOx was included in the anode of an MEA at two different loadings (0.1 mg Ir /cm2 (1:1 Pt/Ir) and 0.06 mg Ir/cm2 (1:0.06 Pt/Ir)). The results showed that the addition of the OER did not compromise the performance of the MEA under normal operating conditions. Furthermore, the MEAs generated proved reversal tolerant capabilities by withstanding multiple, prolonged periods of fuel starvation by maintaining the cell potential in the water electrolysis potential range. The effects of fuel starvation were found to increase the ohmic resistance of the cell, possibly due to membrane dehydration and ionomer degradation. After testing, the MEAs were also characterised using XPS, SEM, cyclic voltammetry to determine the physical changes that occur during these events.