Numerical Modelling of Proton Exchange Membrane Water Electrolysis
Creators: Moore, Michael
Publisher: name: University of Alberta Library
Year: 2024

<p>Proton exchange membrane water electrolysis (PEMWE) is a promising technology in the drive to reach net zero for its ability to generate hydrogen from renewable energy, thereby allowing for large scale renewable energy storage.  However, a number of issues prevent widespread deployment, such as the cost of the iridium based anode catalyst layer (ACL). An improved understanding of the processes occurring in the ACL will be needed to design and optimise the layer, and allow for lower loadings to be used. Recent studies have identified charge transport in the ACL as a limiting factor, particularly electron transport. In addition, the importance of two-phase flow in the anode is an open question, with mass transport effects being difficult to isolate and investigate. In this work numerical modelling is used to provide insights that can guide future research efforts, with a focus on the ACL. </p>

<p>As part of understanding charge transport in the ACL, the protonic conductivity of the layers needed to be measured. While the hydrogen pump technique has been successfully applied to catalyst-free carbon/ionomer layers, there is the possibility of protonic phase being bypassed by a reactive pathway in the unsupported catalyst layers used in PEMWE. Numerical modelling and experimental studies are used to prove the existence of the bypass in platinum/carbon based layers, and to assess its impact in iridium (Ir) and iridium oxide (IrOx) based layers. The high frequency resistance (HFR) is used to isolate the contact resistance between the catalyst layer and the membranes, and parametric studies show the importance of the electronic conductivity, in addition to the activity, of the catalyst layer. Unsupported layers comprised of IrOx were found to be unaffected by the bypass, while Ir black layers were strongly affected.</p>

<p>Next, two-phase flow in the ACL of a PEMWE cell was investigated. The micro-structure of the porous media was captured using a pore size distribution (PSD) model that allowed for the impact of parameters such as pore size and contact angle to be analysed. The model predicts that an Ir based ACL will produce bubbles close to the membrane that can induce very high gas pressures, however very little impact on performance is predicted. Significant displacement of water did not occur due to the pores in the ACL being very small and hydrophilic. Any mass transport losses that are induced in PEMWE cells are therefore likely caused by blanking of large sections of the ACL by bubbles in the PTL, as such it is recommended that experimental studies focus on characterising such events. Finally, the saturation in the PTL was found to be weakly coupled to that of the ACL, due to their very different intrusion curves.</p>

<p>Finally, the impact of the low ACL electronic conductivity was studied. First, the extremely low conductivity reported in the literature was validated using a novel ex-situ experiment that also revealed the dependence of the electronic conductivity on compression. Next, the model was used to breakdown the losses that are induced by the low ACL conductivity, which shows how it adds a significant resistance not captured by the HFR, and potentially adds significant error when estimating Tafel parameters. The model predicts that the catalyst loading of poorly conducting ACLs could be significantly reduced, explaining experimental trends that been reported in the literature. Finally, strategies to mitigate the poor ACL conductivity were explored experimentally, namely increasing cell compression and using electronically conducting polymers. Both improved performance significantly by increasing the active area and reducing the HFR, underscoring just how impactful the poor ACL electronic conductivity is on performance.</p>

<p>In summary, this work sought to understand the impact of charge and mass transport in the ACL of PEMWE cells. This was achieved by first developing a numerical model to validate techniques used to measure the protonic conductivity of ACLs. An electrolyser model was developed that allowed for the parametrisation of the ACL microstructure and therefore understand its impact on two-phase flow. Finally, the model was used to assess the extent to which the ACL electronic conductivity affects cell performance, and discussed methods to mitigate this limitation and improve cell performance.</p>