Commercialization of fuel cell technology for heavy-duty bus application relies on the durability of the components used in fuel cell stack. The durability of polymer electrolyte fuel cell (PEFC) is affected mainly by degradation of catalyst coated membrane (CCM). CCM consist of Pt/C based catalyst layers coated on both sides of PFSA ionomer membrane. In an operating fuel cell, PFSA ionomer membrane degrades under the action of combined chemical/mechanical stresses and Pt/C electrocatalyst degrades due to high voltage excursions. In this thesis, the most relevant approach to understand PEFC degradation during its operation is carried out by employing in situ stressors. The mesoscale morphology and affected physico-chemical properties of fuel cells are investigated with the commonly encountered stressors. Firstly, the mesoscale morphology and its relation to physico-chemical properties of the ionomer membrane under the influence of an accelerated stress test (AST) featuring in situ coupled chemical/mechanical stresses are investigated. The role of combined chemical/mechanical stresses on the ionomer membrane mesoscale morphology and structure is studied using transmission electron microscope (TEM) and thermogravimetric analysis. It is determined that the microstructure of PFSA ionomer membrane is strongly influenced by the degradation history of PEFC. The mesoscale morphological degradation is found to precisely influence the water uptake of the ionomer membrane. The effects realized through chemical and mechanical stressors in coupled and decoupled forms are evaluated through the mesoscale morphology and physico-chemical property studies. Secondly, cathode catalyst layer (CCL) subjected to a voltage cycling AST to mimic the high voltage excursions is studied. It is found that the CCL degradation led to the inhomogeneous distribution of solid and pore phases. The change in the CCL structure accompanied by the platinum agglomeration, carbon corrosion and spatial redistribution of ionomer with voltage cycling is investigated using TEM micrographs with phase sensitive mapping. The observed degradation effects of CCL through the agglomerated and dissolved platinum, corroded carbon, spatially redistributed ionomer, and compacted solids revealed the underlying mechanisms of activation and mass transport losses. Overall, a fundamental understanding of degradation mechanisms in CCM components at mesoscale is achieved from in situ fuel cell testing, which is of particular interest in commercializing and developing durable fuel cells.