The Role of Thermal Conductivity on Liquid Water Distribution in GDLs

Abstract

Operating polymer electrolyte fuel cells (PEFCs) at increasingly higher current density and efficiency necessitates overall improvements in fuel cell water management [1]. A crucial role facilitating these improvements is played by the gas diffusion layer (GDL) within the membrane electrode assembly (MEA) [2]. The GDL is responsible for distributing the reactant gases from the flow field towards the catalyst layer as well as removing reaction products from the catalyst layer to the flow fields. For example, at the cathode, the electrochemical reaction of oxygen ions with protons and excess electrons yields water and heat, which must be removed through the GDL to the flow field. The removal of water through the GDL can lead to blocked pores in the GDL [2], thereby restricting the reactants’ gas flow towards the catalyst layer causing mass transport losses. GDL design is crucial towards improving the performance of PEFCs, especially at high current densities, such that it minimizes these transport losses. Additionally, removal of the excess heat generated at the cathode catalyst layer through the GDL indicates that the GDL thermal conductivity (k) plays a crucial role affecting the temperature distribution within the MEA [3].In this work, two GDLs varying in thermal conductivity were selected and assembled in MEAs which were then imaged in-operando using X-ray tomographic microscopy [4]. In-Operando imaging allowed the evaluation of the liquid water distribution at steady state over a range of operational conditions such as temperature and current density. Water distribution results for a range of current densities will be presented for selected cell temperatures between 40 and 70 °C. For GDL I with the lower thermal conductivity, the channels are completely dry at 40 °C. For GDL II with the higher thermal conductivity, a wet-dry transition was observed in between 50 and 70°C. High saturations were observed in both the channel and the land regions at 50 °C, while at 70 °C the channels appear to be dry with liquid water only being present under the lands (see Figure 1). These results provide experimental evidence of a major influence of GDL thermal conductivity on liquid water distribution and overall water management in fuel cells. Keywords – operando, X-ray tomographic microscopy, GDL, water visualization Acknowledgement Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Ballard Power Systems, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Canada Research Chairs.