Impact of Liquid Water on Oxygen Transport Resistance in a PEMFC Cathode Via Limiting Current Methods and Concurrent Synchrotron X-Ray Radiography

Abstract

The accumulation of liquid water formed by the electrochemical reaction of hydrogen and oxygen in a polymer electrolyte membrane (PEM) fuel cell causes a gas diffusion layer (GDL) to become effectively less porous and more tortuous. To achieve designs directed for water and gas transport in PEMFC diffusion media, a nuanced understanding of the nature and extent of GDL water as an impedance to oxygen flux is required. In this work, a limiting current approach was used to measure the mass transport resistance in a custom PEM fuel cell. Using a small active area at high stoichiometric ratios, it was assumed that the concentration of gases was constant along the length of the reactant flow channels, creating a uniform concentration gradient across the entire diffusion medium. If the fuel cells are then operated at the limiting current density, where cell potential approaches zero, it can also be assumed that all the reactants are being consumed and the concentration of reactants at the reaction site is effectively zero. Consequently, oxygen transport resistance in these conditions can be inferred directly from the limiting current density using the following equation, where RT is the transport resistance in s/cm, c0 is the oxygen concentration at the reactant channel and iL is the limiting current: RT = 4Fc0/iL Previously, through a thorough set of limiting current experiments at a range of operating conditions, Baker et al.1 showed that the carbon-fiber diffusion medium is the dominant contributor to oxygen transport resistance. Owejan et al.2 combined limiting current methods with in situ neutron radiography to demonstrate a strong qualitative correlation between GDL saturation and oxygen transport resistance. Synchrotron radiography has been established as a powerful tool for visualizing water distribution in PEMFC diffusion media3. In this work, synchrotron X-ray radiographs were obtained with a pixel size of 6.5 µm with a spatial resolution of 10 μm. GDL materials were imaged in operando at limiting current over a range of relative humidities (set in the inlet cathode gas stream). Figure 1 (a) is an example radiograph of a cell operating at limiting current density processed according to the Beer-Lambert law5,6. A distinct in-plane variation in liquid water distributions is visible between regions over ribs and regions over channels (Figure 1 (b)). This work exploits the high spatial resolution for capturing liquid water accumulation trends, information which is essential for building towards a direct, quantitative empirical relation between oxygen transport resistance and GDL saturation.