The contribution of hydrogen in the transition to clean energy is significant, especially in the transport, construction and energy sectors.Consequently a great deal of Research is aimed at green production of hydrogen, through processes involving water, such as electrolysis coupled with renewable resources (solar photovoltaic and wind power). The current thesis focuses on the analysis of low-temperature water electrolysis using a proton exchange membrane (PEM) water electrolyzer. In particular, the main idea is to identify the types of electrolyzer losses, described in the literature, using a non-destructive in situ investigation technique: electrochemical impedance spectroscopy (EIS). This technique is able to distinguish the contribution of each overvoltage that contributes in the loss of efficiency of the system by applying a small perturbative signal and measuring its frequency response.