Quantifying Spatiotemporal Heterogeneity within Fuel Cells Using Simultaneous Neutron and X-Ray Tomography (NeXT)

Abstract

Heterogeneity within electrochemical devices, such as fuel cells, influences their performance and durability in ways that are not fully understood. 4-dimensional (4-D; 3 spatial dimensions and time) operando visualization techniques such as X-ray (1-2) and neutron (3) tomography are powerful tools to probe heterogeneity within operating electrochemical devices in high spatiotemporal resolution. Combining neutron and X-ray tomography (NeXT) (3-4) simultaneously offers unique advantages over single-modality methods, especially in terms of enhanced contrast between materials. There is a significant opportunity to utilize NeXT to characterize and quantify heterogeneity within electrochemical devices during cell operation. In this study, we utilize quantitative NeXT to identify spatiotemporal heterogeneity in the morphology of the membrane electrode assembly (MEA) and water distribution within the porous layers and the membrane. A custom interfacial tracking algorithm is utilized to accurately characterize 4-D morphology and boundaries of cell components. Heterogeneity is found to depend upon location with respect to cell components and operating conditions (such as current density and inlet relative humidity). Variations in the MEA morphology is dominated by variations in membrane morphology, whereby variations of up to 80 μm is observed in membrane thickness for N117 membranes (Chemours, USA). We find that the interface between the gas diffusion layer and the enclosing gasket is a high porosity region that accumulates liquid water during cell operation, and this liquid accumulation leads to high membrane hydration and membrane swelling near the interface. With this study, we demonstrate the viability of quantitative NeXT to characterize operando heterogeneity within multi-component electrochemical devices, taking us a step closer towards rational control and design of inherent heterogeneity within these devices.