Membrane Electrode Assembly Fabrication Method Effects on Catalyst Layer Structure, Interfaces, and Performance

Abstract

The membrane electrode assembly (MEA) is a critical component in polymer electrolyte membrane fuel cells. MEAs are often prepared by depositing the catalyst layer directly on to the membrane. In lab-scale research, catalyst coated membranes or CCMs can be prepared by hand painting or ultrasonic spraying. Catalyst-coated diffusion media (CCDM), also known as gas diffusion electrodes (GDE) have the catalyst layer deposited on the gas diffusion media. For the large-scale manufacturability of MEAs, GDEs may be easier to manufacture in a roll-to-roll (R2R) fashion because the challenges associated with membrane swelling are avoided. In addition, for R2R fabrication, there are many ink deposition methods. For example, in gravure coating, a patterned gravure roller can be used in combination with a doctor blade and a pressure roller to coat the catalyst layer. The coating quality can be optimized by modifying the rotation speed of the gravure roller.1 Alternatively, in slot die coating modification of the distance from the slot die to the substrate can be used to improve coating quality, effectively changing the sheer rate experienced by the ink. With all fabrication methods (ultrasonically sprayed, slot die R2R, micro gravure R2R, etc.) several processing parameters can be varied, impacting the structure of the electrode and its performance. Additionally, ink composition (solvent ratio, ionomer to catalyst ratio) will also have significant impact on the catalyst layer structure.2 Thus, the MEA fabrication method, the processing conditions, ink composition, and chemistry and morphology of the catalyst will all affect the surfaces and interfaces that exist in an MEA. It is important to understand these material interfaces and the structure, property, performance relationship between the catalyst layer components. Electron microscopy plays a key role in these studies. This work will highlight the importance of assessing MEAs at different scales and from different perspectives: catalyst morphology, top-down electrode view, cross-sectional MEA view. Microscopy can be a powerful tool for visualizing all the components within an electrode structure. Extraction of quantifiable parameters from digital image processing provides direct correlations with performance metrics, furthering the utility of electron microcopy for electrode studies. [1] S.A. Mauger et. al, Gravure Coating for Roll-to-Roll Manufacturing of Proton-Exchange-Membrane Fuel Cell Catalyst Layers, J. Electrochem. Soc. 165 (2018) F1012–F1018. doi:10.1149/2.0091813jes. [2] Khandavalli, S. et al. Rheological Investigation on the Microstructure of Fuel Cell Catalyst Inks. ACS Appl. Mater. Interfaces (2018). doi:10.1021/acsami.8b15039 Figure 1