Pairing Gas Diffusion Media for High-Power PEMFC Operation

Abstract

Proton exchange membrane fuel cell (PEMFC) water management and current density are highly sensitive to the gas diffusion media (GDM) used at both the anode and the cathode. As the gas and mass transport requirements vary at each of these electrodes, the optimal properties for the respective GDM contrast. Utilizing different GDM at the anode and cathode, in an asymmetric GDM pairing, maximizes current densities in a broad range of operating conditions compared to symmetric pairings comprising the same GDM at the anode and cathode. The optimal asymmetric GDM pairing features a highly permeable anode GDM with a high porosity and thick microporous layer (MPL, 80 µm) that is homogeneous in through-plane flux. Alternatively, the cathode GDM is less permeable and has a thinner and less porous MPL (20-45 µm) with a broad flux distribution. Net water drag measurements show that this asymmetric GDM pairing facilitates water retention at the anode and water expulsion from the cathode to realize lower oxygen transport resistance and up to 73% higher current density than PEMFCs with symmetric GDM pairings. Symmetric GDM pairings are insufficient at removing product water from the PEMFC cathode, resulting in greater cathode water saturation. As a result, more product water must back-diffuse across the membrane to be removed from the anode at the expense of significantly greater oxygen transport resistances and concomitantly lower current and power densities than the asymmetric GDM pairing. GDM are characterized by X-ray computed tomography (CT) and scanning electron microscopy (SEM) to reveal differences in MPL carbon type, average pore size, pore character, porosity, pore connectivity, tortuosity, and spatial flux distribution that govern PEMFC gas transport and water management.