Multi-Technique Characterization of Gas Diffusion Fuel Cell Electrodes

The development of large-scale manufacturing capabilities for the most critical component in polymer electrolyte membrane fuel cells (PEMFC), the membrane electrode assembly (MEA) is essential for the commercialization of this technology. Most commonly, the catalyst is combined with a solvent to make an ink, which is then deposited onto the polymer membrane, producing a catalyst coated membrane (CCM). Gas diffusion electrodes (GDEs), in which the catalyst layer (CL) is deposited directly on the gas diffusion media (GDM) are easier to manufacture in a roll-to-roll (R2R) fashion. Additionally, some of the issues associated with polymer electrolyte membrane swelling that are observed during CCM fabrication are eliminated with R2R GDEs. While significant work has been done to study the differences in initial performance for CCM based MEAs further studies are needed on electrodes made with scalable routes. This work focuses on comparing several spectroscopic characterization methods that were applied to a set of GDEs to better understand the bulk and local ionomer distribution and its correlation to initial performance metrics. The GDEs were made with a state-of-the-art Pt catalyst supported on high surface area carbon, a 0.9 ionomer to carbon ratio, and a 1-propanol and water ink solvent mixture. The GDEs in this study were fabricated by an ultrasonic spraying system, as well as, by slot die and gravure roll-to-roll coating methods. Various ink solvent ratios were used, and in the case of the sprayed electrodes the incorporation of additional ionomer on the GDE surface was explored. The F/Pt ratio of all the GDEs was measured by x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) probing the difference in the ionomer amount at different depth (around 10 vs 100 nm). Additionally, scanning transmission electron microscopy (STEM-EDS) analysis of the electrode cross-sections was performed at low magnifications to measure F/Pt ratio within the entire thickness of the electrode as well as at a higher magnification to probe the local F/Pt ratio inside of the catalyst layer (CL). The F/Pt from the three techniques was then correlated with the initial performance metrics of potential at 1 A/cm2 and at 1.5 A/cm2, as well as, CL resistance, and electrochemically active surface area (ECSA). It was found that values extracted from both SEM-EDS and XPS show similar correlations that track with the initial performance values. The R2R slot die GDE with a high water ink content showed the highest F/Pt at the surface of the electrode and demonstrated the best performance. A certain range of ionomer content present at the electrode surface was shown to induce a more favorable Cl/membrane interface improving the fuel cell performance. This work highlights the importance of assessing the composition and structure of fuel cell electrodes at different surface depths and builds towards identifying key metrics that correlate to performance that can be derived quickly to explore a larger space of processing and fabrication parameters.