Liquid Water Uptake and Contact Angle Measurement of Proton Exchange Membrane Fuel Cell (PEMFC) Electrodes

Abstract

Proton exchange membrane fuel cells (PEMFCs) are promising power sources for various stationary and portable applications. Despite the numerous efforts made the recent years, water management remains a major limiting factor to PEMFC performance and durability especially for ultra-thin catalyst layers (CLs) [1]. Therefore, a basic understanding of the wetting and morphological properties of the CLs is crucial to overcome mass transport limitations. This requires measuring and quantifying several properties including contact angle and ionomer water uptake. In this work, imbibition tests were performed to extract (external) contact angle to water and ionomer liquid uptake of free-standing electrodes (about 8 mm thick) fabricated by the 3M Company. They consisted of Vulcan™ XC-72 carbon (without catalyst) dispersed in a 3M brand ionomer matrix with an equivalent weight (EW) of 825 g/mole. Not stiff enough, these films were first hot-pressed on both sides of an Ethylene Tetrafluoroethylene (ETFE) film (120 μm thick) before being immersed 10 mm into liquid (deionized) water for 900 seconds (see Figure 1.a). The increase in sample mass due to the added liquid weight was determined with respect to time during the measurement using a tensiometer (model K100 from Krüss GmbH, Germany) operated at room temperature. After 900 seconds, the samples were pulled out the liquid to estimate the wetting force and thus the external contact angle to water using the Wilhelmy equation. This approach was described with more details in our previous work [2]. The Figure 1.b shows the time evolution of the gain in mass for carbon-ionomer films with ionomer to carbon (mass) ratios of 0.8, 1.0, 1.2 and 1.4 when immersed 10 mm into liquid water. The amount of absorbed water significantly increased with the ionomer presence. The Figure 1.c shows the (external) contact angle to water as function of the I/C ratios estimated by calculating the capillary force when the samples were pulled out the water at t=900s (method described in [2]). The results reflected a highly hydrophobic behavior of the carbon-ionomer films: average contact angle to water about 140°. These observations were confirmed by optical measurement using the sessile drop technic. The electrode hydrophobicity was mainly ensured by the solid phase and the contact angles were little sensitive to the amount of ionomer (for the material and range of I/C considered in this work). Material with different ionomer were also used to determine the influence of the ionomer chemistry on the electrode liquid uptake and contact angle to water. The effect of the temperature on electrode wettability was also investigated. Figure 1. (a) Krüss tensiometer K100 (Krüss GmbH, Germany) used to measure the mass evolution of carbon-ionomer films after being immersed 10 mm into a liquid (at a constant speed of 100 mm/min). The carbon-ionomer films were hot-pressed on both sides of an ETFE film (120 μm thick) with a total surface of 3 x 2 cm2. (b) Time evolution of the gain in mass for carbon-ionomer films (Carbon Vulcan™ XC-72, 3M brand ionomer matrix with EW 825) with I/C ratios of 0.8, 1.0, 1.2 and 1.4. (c) Contact angle to water as function of the sample I/C ratios determined by calculating the capillary force when the samples were pulled out the water at t=900s (see [2]). The contact angles calculated from this approach were in good agreement with the ones measured using the sessile drop technic.