Investigations on liquid water distribution in polymer electrolyte fuel cells

Effective management of liquid water is crucial for enhancing the performance and longevity of fuel cells, particularly at high power density operation. The membrane electrode assembly, comprising a porous diffusion medium and polymer membrane, is central to cell operation. However, flooding caused by the presence of liquid water from either humidification or electrochemical generation can impede reactant flow. This research contributes to comprehending how liquid water is distributed within the fuel cell, the factors that influence it, and its implications for performance and longevity. A novel visualization method was developed for observing both steady-state and transient liquid water breakthrough from sequential radiographs. This revealed that condensation initially appeared around the flow channel lands, signifying a transition from vapour to capillary-dominated water removal. Using two- and three-dimensional (2D and 3D) operando X-ray tomography datasets and a new understanding of liquid water instability through tomography grayscale value, this study found a strong correlation between the spatial distribution of liquid water and GDL pore structure. Liquid water saturation pathways are shown to get increasingly defined closer to the flow channel, consistent with the capillary mechanism. Correlating pore structure with 2D and 3D saturation data revealed different pore characteristics that explained residual saturation, leading to significant lateral flow, and those that promoted rapid eruptive liquid water breakthrough. To further generalize the understanding of factors influencing liquid water distribution, the effects of the internal thermal gradients on liquid water distribution was also examined. It was demonstrated that thermal gradients resulting from heat generation from overpotential and heat distribution due to GDL configurations were significant contributors to through-plane liquid water distribution and vapour phase water removal within the GDL. Lastly, investigating the interactions between cathode catalyst layer degradation and liquid water distribution revealed that increased heat generation due to degradation caused increased vapour phase removal, especially at high current density. It was also shown that the presence of liquid water near the catalyst layer could significantly accelerate the degradation process, which can be mitigated through targeted GDL design.