The proton exchange membrane fuel cell (PEMFC) is an important technology for clean power generation in a decarbonized hydrogen system and is notably envisioned for applications in heavy-duty transportation. However, for cost and performance competitiveness, improvements are still required in efficiency and durability. Optimization of the complex structure of multicomponent cathode catalyst layers (CL), where the oxygen reduction reaction (ORR) takes place, is a promising strategy to conciliate low mass transport resistances, high kinetics, and good performance retention. Yet, much of the morphology of this layer remains poorly known. This includes the interactions of the ionomer network, the nature of the micropores, the influence of the porosity on performance losses and calls for advances in characterization of the CLs at the nanoscale. In this thesis, I used advanced transmission electron microscopy (TEM) methods to study these questions in CLs fabricated with Pt nanocatalysts dispersed on (porous) carbon supports and perfluorinated sulfonate acid ionomers. Electron tomography (ET) at cryogenic temperature was first used to gain three-dimensional (3D) insights into the preserved morphology of intact CLs. By operating at a low electron dose and using advanced image processing methods for denoising and segmentation, accurate volumetric reconstruction with limited electron beam-induced degradation was achieved. The results reveal the intricacy of the ionomer morphology at the nanoscale and show that this is a highly continuous network with remarkable thickness heterogeneities which, furthermore, connects all Pt catalysts at the surface of the supports. Next, to gain further insights into the interior microporosity of the carbon supports, a sample preparation method enabling full-range, high-resolution ET was established. The reconstructions uncover typically large interior mesopores with few-layers carbon walls, separated from the external CL pores by compact carbon shells in which rare and mostly sub-nm pores are seen. Finally, to ultimately understand how this porosity influences degradation pathways in the CL, progress is detailed towards the study of Pt/C catalysts in real time with electrochemical liquid phase (ec-LP)TEM. As a whole, the knowledge gathered herein offers a more accurate 3D picture of the CL in PEMFC and pathways towards their operando characterization at the nanoscale. A such, this thesis shows how multidimensional TEM can aid the development of improved PEMFCs.