The current energy crisis demands the development of new sustainable energy sources and devices with low environmental impact, including in the field of electrochemical cells. Mass transport limitations can reduce the overall performance of the device by limiting the flux of reactants towards the electrodes. The optimization of mass transport in electrochemical devices can be achieved by designing appropriate gas diffusion electrodes (GDE) that maximize reactants diffusion and minimize undesired processes such as liquid water saturation in proton exchange membrane fuel cells (PEMFC). Gas diffusion electrodes in PEMFC consist of a gas diffusion layer (GDL) bearing a catalyst layer (CL). The GDL is further divided into a macroporous substrate (MPS) and a microporous layer (MPL). The fundamental understanding of gas and water transport in GDEs requires a detailed knowledge of their pore network structure, which encompasses pore sizes in the range of tenths of nanometers in the CL and MPL. As such, electron imaging represents a fundamental tool in characterizing GDEs. This thesis aims to provide new insights into the analysis of diffusion media with electron microscopy, including focused ion beam-scanning electron microscope (FIB-SEM) tomography to investigate the 3D structure of the pore network, and environmental scanning electron microscopy (ESEM) to observe liquid water behaviour. The study focuses on the characterization of constrictions size and effective tortuosity as MPL descriptors, and the monitoring of liquid water condensation in catalyst layers (CLs) and microporous layers (MPLs) with different structural parameters and surface hydrophobicity. The results highlight the key role of constrictions size in water evacuation, the fundamental contribution of narrower size pore connectivity increasing the effective tortuosity in hydrophilic MPLs, and the effect of structural parameters on water condensation. Ultimately, the design of a novel fuel cell operando ESEM stage is proposed.