Parametric Study of Fabrication Conditions for High-Performance Gas-Diffusion-Electrode-Based Membrane-Electrode Assemblies

Abstract

In polymer electrolyte membrane fuel cells (PEMFCs), the membrane electrode assembly (MEA) is the critical component for power production as it is responsible for the electrochemical reactions that convert the fuel to power. The two most common MEA architectures are the catalyst-coated membrane (CCM) and the gas-diffusion electrode (GDE), also referred to as catalyst-coated diffusion media (CCDM). GDEs are of interest for MEA fabrication because under some fabrication and/or operating conditions GDE MEAs have shown improved performance over CCM MEAs.[1] GDEs may also have advantages for roll-to-roll manufacturing due to more robust mechanical properties of the diffusion media and avoidance of membrane swelling during catalyst layer coating. However, the challenge with GDEs is that the catalyst layer/membrane interface is usually inferior to that of CCMs, leading to inferior performance. It has been previously demonstrated that adding a layer of ionomer on top of the GDE catalyst layer can improve the performance of GDE-based MEAs [2], though most studies still show performance inferior to that of CCM-based MEAs. Building on this foundation, our work comprehensively investigated and optimized the fabrication and processing conditions (ionomer overlayer thickness, hot pressing) required to produce high performance GDE-based MEAs with performance comparable to CCM-based MEAs. It was demonstrated that coating a thin ionomer overlayer and then hot pressing the GDEs to the membrane could achieve comparable catalyst activity and air performance to CCM MEAs. Atomic force microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) combined with Energy Dispersive X-Ray spectroscopy( EDS) were employed to investigate the MEA structure including the ionomer overlayer, and interfaces of GDL/catalyst and catalyst/membrane. In situ performance testing was used to assess the impact of MEA fabrication conditions on catalyst utilization and air current-voltage performance. Modeling of electrochemical impedance spectroscopy measurements showed lower catalyst layer resistances in the MEAs with the ionomer overlayer, which was in good agreement with the electrode structure observations. The critical amount of ionomer required to achieve maximized performance was defined. The mechanism for improved performance from the ionomer overlayer and hot pressing were proposed and evidenced: 1) adhering the GDE to the membrane to increase the interfacial contact area of catalyst layer/membrane; and 2) creating a smooth GDE surface with a better contact between GDE and membrane. Moreover, the enhancement of the performance by ionomer overlayer was demonstrated to be highly related to the surface roughness of the gas diffusion media (GDM). The GDM with a smoother surface required less amount of the overlayer ionomer than a GDM with a rougher surface to achieve comparable fuel cell performance.