Beyond a shadow of a doubt, our generations are witnessing the energy revolution, in which fossil fuels are gradually being replaced by renewable energy sources. In that sense, proton exchange membrane fuel cells (PEMFCs) accompanied by batteries, play a key role in establishing fully electrified transportation and replacing conventional internal combustion engine vehicles. To complete the water cycle, PEMFCs will also have to be powered by using 'green' hydrogen produced from water using electrolysis where the electrical energy is sourced using emissions-free sources such as wind and solar energy. However, while establishing supply, storage and transportation is a grand challenge by itself, PEMFCs as a 'down-stream' technology for hydrogen use also have to solve many major bottlenecks to keep up with the massive scalability of hydrogen technologies in the next decade. Namely, one of the largest such bottlenecks can be found in the membrane electrode assembly (MEA)the Pt-based electrocatalyst. Although the electrocatalyst is essential for both the anodic hydrogen oxidation reaction (HOR) and the cathodic oxygen reduction reaction (ORR), the cathode requires many times higher amount of the Pt-based electrocatalyst than the anode resulting from significantly slower kinetics of the ORR. On the other hand, although there are various very important studies about non-precious group metal (non-PGM) electrocatalysts, in reality the only electrocatalyst applied at the production level for PEMFCs used for transport applications is still the Pt-based electrocatalyst. Due to a very high price of deficient Pt, it represents almost half of the total costs of the PEMFC system manufacturing, even when considering the economies of scale. One of the most promising approaches to reduce both the quantity of Pt per vehicle as well as the costs related to platinum per PEMFC system is to alloy Pt with a less noble and at the same time less expensive metal (M = Co, Cu, Fe or Ni), which enables better Pt utilization, i.e. a higher electrochemically active surface area, due to the dilution of Pt-atoms inside the nanoparticles (NPs) core, and a higher activity toward the ORR due to the combination of a ligand, strain, coordination number, and/or surface disorder effects. Nevertheless, while the activity of carbon-supported Pt-based nanoalloys is quite well understood, the long-term durability of these electrocatalytic systems is still holding many key open questions. Specifically, the fuel cell electrocatalysts are exposed to very aggressive conditions, including a corrosive environment, high temperatures, humidity, frequent stops and starts, fluctuations in the operational voltage, and so forth. These non-intrinsic operational conditions, together with the intrinsic properties of the electrocatalyst (choice of M, order/disorder, de-alloying/activation, type of carbon/degree of graphitization ), lead to a spectrum of very complex and interconnected degradation mechanisms: (i) electrochemically induced (transient) dissolution of Pt, which is defined by thermodynamic tendency of Pt to the formation/reduction of the Pt-oxide, resulting in Ostwald ripening and/or formation of metallic Pt-bands in the membrane, (ii) dissolution of M and (iii) electrochemical and chemical carbon support corrosion, leading to the agglomeration and/or detachment of whole Pt NPs. These complex instability phenomena are of crucial importance for the longevity of the PEMFCs in the case of their application in both light-duty vehicles (LDVs) beside heavyduty vehicles (HDVs). In particular, as a plan for additional cost reduction, the target for the electrocatalyst loading for LDVs vehicles, proposed by the US Department of Energy (DoE) and expected to be achieved by 2025, is less than 0.1 mgPt cm -2 . However, such a low loading of the electrocatalyst can also directly increase its instability and finally significantly affects the overall performance of the fuel cell over time. While targeted electrocatalyst loading might not affect the stability of the electrocatalyst in the HDVs to the same degree (0.25 mgPt cm -2 ), on the other hand, the average travel distances, and consequently the lifetime requirements of the HDVs, are significantly higher than for the LDVs. In particular, the current goal for longevity of the PEMFCs (expected to be achieved by 2030), proposed by the US DoE, is an operation time of 5000 hours for the LDVs (with an ultimate goal of 8000 hours, expected to be achieved by 2050) and 25 000 hours for the HDVs (with an ultimate goal of 30 000 hours, expected to be achieved by 2050), with less than 10% loss in performance. Therefore, a comprehensive understanding of the degradation mechanisms of the carbon-supported Pt-based nanoalloys is essential. Here, the gap between knowledge obtained at levels of the rotating disc electrode (RDE) level and the MEA must be overcome.