Quantifying Spatio‐Operational Heterogeneity in Electrochemical Devices via Operando Correlative Neutron and X‐Ray Tomography

Abstract Heterogeneity in component morphology and distribution, inherent in modern electrochemical devices, frequently limits device performance and durability. However, accurately characterizing heterogeneity is challenging as it requires high‐contrast detection of evolving multi‐material components and associated interfaces, and this often bottlenecks rational design. In this study, new insights into spatio‐operational heterogeneity are quantitatively revealed within multi‐component electrochemical systems using simultaneous neutron and X‐ray tomography (NeXT). In operando fuel cells, this technique uniquely offers independent yet simultaneous and correlated characterization of material distribution and morphology. This enables accurate contextualization of liquid water within all key component interfaces in sufficient detail to resolve previously unidentified 4D heterogeneity. First, 4D heterogeneity in membrane thickness and water content is found to depend strongly upon location and operating conditions, with membrane thickness variations up to 80 µm and membrane water content variation from dry to hydrated at 21 . Second, a direct experimental link is established between anisotropic humidification and local anisotropic swelling of the membrane. The observations lend unique insights into degradation mechanisms of the membrane and have notable implications on the practical durability of fuel cells. The proposed methodology is highly relevant to advancing multi‐material electrochemical devices (with evidence of applicability to batteries provided).