The commercial viability of polymer electrolyte fuel cells (PEFCs) has increased rapidly over recent years with applications in public and commercial transportation, back-up power, and un-manned autonomous systems. This has come as a direct result toward increasing evidence and severity of climate change due to greenhouse gas emissions;​ pushing the need for government regulations to introduce stricter limits on fossil fuel combustion in new passenger cars, as well as in other light-to heavy-duty vehicles. Further cost and durability improvements in PEFCs present significant opportunities as the technology continues to be refined. PEFCs are assembled as a series of layers, each having specific functionalities to optimize the cell performance during electrochemical conversion of chemical potential energy, in the way of hydrogen and oxygen, into useable electrical power, heat, and water. These PEFC materials can undergo considerable changes during operation, and lifetime testing through critical degradation processes, which can be uniquely captured using X-ray Computed Tomography (XCT) in this complex multi-layered system. XCT provides a unique ability to delve into the innermost structures through non-destructive imaging in diverse and extensive application areas. In this thesis, a novel small-scale fuel cell fixture that mimics the performance and degradation features of a full-scale PEFC assembly is presented. By combining the 3-dimensional visualization through repeated identical location tomography using XCT scans at various temporal stages of this small-scale fixture, powerful in-situ and operando investigations of dynamic material properties are obtained. This methodology is termed as 4D CT. By means of applying accelerated stress tests focused on cathode catalyst layer degradation, unique insight into the lifetime, dynamics and interactions between the catalyst layer and surrounding components was uniquely obtained using custom developed tools and analysis methods. These new methods allow for new investigations into the temporal changes of water saturation and cathode catalyst layer morphology. It has been found that during ageing, the morphological interaction between different layers can have a considerable impact on degradation mechanisms such as crack propagation. These results uncover unique evidence around the strongly interactive nature of material degradation within a fuel cell that has previously been unobserved.