Pairing Asymmetric Gas Diffusion Media for High-Power Fuel Cell Operation

Abstract

Proton exchange membrane fuel cell (PEMFC) power production is highly influenced by the properties of the gas diffusion media (GDM). The GDM typically comprise a mesoporous carbon layer (MPL) on top of a porous carbon-fiber gas diffusion layer (GDL) which are both wetted with polytetrafluoroethylene (PTFE) to modify their water retention. In a PEMFC, the GDM are compressed to either side of the catalyst coated membrane. The GDM are active in the fuel cell transport processes including electron and heat conduction, water removal, and distributing reactant gases to the catalyst layers, which affects the electrochemistry in the catalyst layers. These transport phenomena rely on both the properties of the GDM solid phase and its voids, making optimization of GDM material properties challenging. The majority of reports in the PEMFC literature addressing GDM focus solely on the cathode, since water is produced at this electrode and its ineffective removal can block active sites and occlude O2 diffusion. Most research reports use symmetrical anode and cathode GDM. To date, there has been less discussion on the role of the anode GDM or how anode and cathode GDM properties can be selected in concert to increase fuel cell power output, despite that a significant amount of water in a fuel cell is rejected to the anode.We consider that anode and cathode GDM properties, such as air permeability, must be chosen in unison to improve cell water management. In our previous work, we used X-ray computed tomography to observe that dry-laid, non-woven, Freudenberg GDM maintain large void volume while permitting high compressive stress, ultimately resulting in lower contact and ohmic resistances and better mass transport compared to another distinct class of GDM (wet-laid, SGL).1 In this work, we make a range of PEMFCs using a suite of Freudenberg GDM to study the influence of anode and cathode GDM air permeability and how these properties may be paired to improve cell hydration and increase fuel cell power production in a broad range of operating conditions. Fuel cells are characterized with cyclic voltammetry, electrochemical impedance spectroscopy (EIS), gravimetric analysis, and limiting current measurements to quantify the sources of mass transport resistance. Gradients of GDM material properties from anode to cathode improve cell hydration and facilitate water removal to reduce mass transport resistances and enable higher power operation. 1. “The Role of Compressive Stress on Gas Diffusion Media Morphology and Fuel Cell Performance,” R. W. Atkinson III, Y. Garsany, B. D. Gould, K. E. Swider-Lyons, I. V Zenyuk, ACS Applied Energy Materials, 2017, 1 (1), 191–201.