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Catalyst layers (CLs) in proton-exchange-membrane fuel cells (PEMFCs) facilitate electrochemical reactions and therefore play a critical role in cell performance. Absorption and desorption of water into both the CL ionomer and the CL pore structure are integral aspects of PEMFC water management and performance. In this work, the water uptake from both the vapor and liquid phases is examined experimentally. Specifically, the dynamic water-uptake behavior of the CL ionomer is investigated as a function of relative humidity, temperature, Pt-loading and pretreatment. The water content of the ionomer in the CL, even after pretreatment, is found to be significantly lower than that for the bulk ionomer membrane, yet with similar sorption time constants. Thus, there is probably substantially slower transport into the ionomer which is likely due to its interfacial character. From the liquid phase, measured capillary pressure -saturation relationships show that the CL has an appreciably hydrophilicity that is strongly dependent on the existence of cracks. These findings are critical to the understanding and optimization of water management and transport phenomena within PEMFCs.

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Water uptake profiles of proton-exchange-membrane fuel-cell catalyst layers are characterized in the form of capillary-pressure saturation (Pc-S) curves. The curves indicate that the catalyst layers tested are highly hydrophilic and require capillary pressures as low as -80 kPa to eject imbibed water. Comparison of materials made with and without Pt indicates a difference in water ejection and uptake phenomena due to the presence of Pt. The addition of Pt increases the tendency of the catalyst layer to retain water. Dynamic vapor sorption (DVS) is used to characterize the water-vapor sorption onto Nafion, Pt/C, and C surfaces. The DVS results align with the trends found from the Pc-S curves and show an increased propensity for water uptake in the presence of Pt.The effect of the ion in Nafion, sodium or protonated form, is also compared and demonstrates that although the protonation of the Nafion in the catalyst layer also increases hydrophilicity, the effect is not as great as that caused by Pt. IntroductionAchieving proper water management in proton-exchange-membrane fuel cells (PEMFC) is rife with challenges. Cathode-catalyst-layer (cCL) flooding is of one concern due to the already slow kinetics of oxygen reduction. Characterization of CL water-uptake profiles enables a fundamental understanding of the driving forces behind flooding and facilitates targeted improvement. However, to date, there has not been data on the wettability and water-uptake behavior of CLs. In this paper, we present results from characterization techniques including capillary pressure vs saturation (Pc-S) curves[1] and ionomer water content measurements taken from DVS. Materials20 wt% Pt/C (5:2 Pt/C:Nafion) CLs were made in-house using an ink painting process and applied directly onto polytetrafluoroethylene (PTFE) membranes from Sartorius Stedim with 0.2 m pores. Successive layers of catalyst ink were added to the PTFE membranes and dried at 140 °C between coatings. Coats were added until the catalyst layer began to show signs of cracking which occurred typically around 20~40

X-ray computed tomography was used to visualize the water configurations inside gas diffusion layers for various applied capillary pressures, corresponding to both water invasion and withdrawal. A specialized sample holder was developed to allow capillary pressure control on the small-scale samples required. Tests were performed on GDL specimens with and without hydrophobic treatments.