Investigation of Spray and Roll to Roll Coated Gas Diffusion Electrode Surface & Bulk Properties Using Microscopy and Spectroscopy

Abstract

Further development of large-scale manufacturing capabilities for the most critical component in polymer electrolyte membrane fuel cells (PEMFC), the membrane electrode assembly (MEA) is needed. Advances in MEA manufacturing capabilities could reduce one of the major cost factors associated with MEA production, the platinum group metal (PGM) catalysts currently used at the cathode electrode for PEMFCs. While spray coating methods have been prevalently used at lab-scale for MEA fabrication, it is desirable to move towards roll-to-roll (R2R) techniques to enable scalable production. Gas diffusion electrode (GDE) based MEAs are easier to manufacture in a R2R fashion than catalyst coated membrane (CCM) based MEAs, since the catalyst layer is deposited directly on the gas diffusion media (GDM) without inducing the PEM swelling observed via other fabrication routes.1 Understanding interfacial properties of these electrodes is important for optimization of electrochemical performance and fabrication procedures employed for wide-scale manufacturing of PEM electrodes.Key surface properties such as surface morphology and surface chemistry, as well as the bulk electrode nanostructure and composition were analyzed for a variety of GDEs with catalyst layers (CLs) prepared by R2R and spray coating methods using X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Energy Dispersive Spectroscopy (EDS). The R2R-coated GDEs were fabricated using the slot die and microgravure coating methods on the same GDM substrate (Freudenberg H23C8) and they were made with both alcohol- and water-rich ink solvent ratios. The ultra-sonic spray coated GDEs were fabricated using both SGL 29BC and H23C8 GDM. In some cases, the spray coated GDEs had an additional amount of ionomer added, referred to as ionomer overlayer.2,3 The water-rich slot die R2R-coated GDE and the spray-coated GDE with an additional ionomer overlayer achieved the highest performance from this set of GDEs. Additionally, these GDEs showed the highest ionomer content measured by both XPS and SEM-EDS and shared a similar level of low surface roughness, implying the differences in composition and performance result from the different coating processes. Interestingly, STEM-EDS didn’t reveal a higher ionomer concentration in these samples, suggesting that these fabrication methods enrich the top surface layer without a significant impact on the bulk electrode. It was also found that the different microporous layer (MPL) surface properties inherent to a specific GDM influence the electrode morphological properties such as surface roughness as well as surface chemistry. The differences in MPL porosity lead to a unique surface ionomer enrichment degree and morphology at the CL surface that will interface with the membrane. These CL surface properties that are in contact with the membrane are hypothesized to be the reason for the improved performance metrics despite fabricating the GDE based MEAs under the same conditions. This work improved the knowledge of how a change in just one material component, or varying the fabrication process, can lead in some cases to vast differences and in others to subtle changes in initial performance, while also demonstrating the effectiveness of multiple characterization techniques towards identifying processing-property-performance trends. Mauger, S. A., Neyerlin, K. C., Yang-Neyerlin, A. C., More, K. L. & Ulsh, M. Gravure Coating for Roll-to-Roll Manufacturing of Proton-Exchange-Membrane Fuel Cell Catalyst Layers. Journal of The Electrochemical Society 165, F1012–F1018 (2018).Wang, M. et al. Impact of Microporous Layer Roughness on Gas-Diffusion-Electrode-Based Polymer Electrolyte Membrane Fuel Cell Performance. ACS Applied Energy Materials 2, 7757–7761 (2019).Mauger, S. A. et al. Fabrication of high-performance gas-diffusion-electrode based membrane-electrode assemblies. Journal of Power Sources 450, (2020).