Improving Utilization of Ir-Based Catalyst Layers in Proton Exchange Membrane Water Electrolyzers
Creators: Beaulieu, Eric J.
Publisher: name: University of Alberta Library
Year: 2024

<p>In order to store large quantities of intermittent renewable energy, proton exchange membrane water electrolyzers (PEMWE) can be used to produce hydrogen which can then be used in a fuel cells to retrieve the energy. One limitation of PEMWE is that the catalysts used, commonly platinum and iridium, are scarce and expensive. In this work, two methods are studied to reduce the loading of iridium. The first method is to reduce catalyst loading of a state-of-the-art IrOx catalyst. Modelling predicted that, for a catalyst with poor electronic and protonic conductivity and high catalyst activity, reducing the loading should result in increased performance. The effects of the reduced loading will be studied to determine catalyst utilization and, if it is poor, study why that is the case. The second method is to combine nickel with iridium to increase the efficacy of the catalyst which, for similar loadings, should provide increased performance. </p>

<p>To test both methods catalyst coated membranes (CCMs) were made using inkjet printing. Ir8NiOx was synthesized in-house using an alkaline aqueous reaction while commercial IrOx, by Tanaka Kikinzoku Kogyo (TKK), was used for comparison and the loading study. To print the anode, Ir8NiOx inks were made by combining propylene glycol (PG), water, and Nafion while IrOx inks contained PG, isopropanol (IPA) and Nafion. To print the cathode, platinum supported on carbon inks were made similarly to the IrOx. The density, viscosity and particle size of each ink were measured to ensure that the fluid properties were compatible with the inkjet printer. These properties are important because they affect the ability of stable droplets to form as well as if clogging of print nozzles will occur. Four CCMs of Ir8NiOx at about 1 mg/cm2 and 4 CCM batches of IrOx at about 0.25, 0.5, 0.75 and 1 mg/cm2 were successfully printed; each IrOx batch includes one of each loading. Testing in a membrane electrode assembly (MEA) was conducted on three of the four Ir8NiOx and 12 of the 16 IrOx CCMs. The remaining CCMs were used only for characterization of the catalyst layer through scanning electron microscopy and energy dispersive X-ray spectroscopy.</p>

<p>As the loading of the IrOx catalyst layers decreased, the i-V curve performance was similar for the first two of the three tested batches. However, the third tested batch showed the 0.5 and 0.75 mg/cm2 cells were similar but the high (1 mg/cm2) and low (0.25 mg/cm2) loadings had better and worse performance, respectively. These results partially validate the previously mentioned model. The benefits of reduced loading were not as drastic as predicted since the increase in kinetic losses was made up for by decreased ohmic losses. Since the effect of decreased loading was not as drastic as the model suggested, electrical conductivity testing was done under compression from 0-95 psig in an attempt to mimic the in-cell compression. It was found that the higher the compression, the higher the electrical conductivity but it did not match measurements of the electrical conductivity of IrOx TKK in literature. This could simply be due to the different method of measurement so, further investigation into the electrical conductivity of the IrOx catalyst is required to determine the cause of this discrepancy. </p>

<p>Ir8NiOx cells were found to have worse performance on an i-V curve as well as low electrochemical active surface area (ECSA) when compared to the IrOx cells. The lower performance was primarily due to the instability of the Ir8NiOx catalyst layer as through SEM imaging it was found that a significant portion of the catalyst left the layer likely due to delamination caused by trapped oxygen bubbles. Additionally, low ECSA could be due to the Ir8NiOx catalyst having a lower surface area, the catalyst layer having a higher packing density, or the catalyst not having as rough of a surface when compared to IrOx. Although overall performance was low, when normalized by the ECSA it was found that the Ir8NiOx performed better on a site to site basis. In addition, the kinetic activity was found to be similar to the IrOx however the ohmic and mass transport losses were larger resulting in the performance loss described above.</p>