Experimental and modeling investigation of low temperature electrolysis through Electrochemical Impedance Spectroscopy

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. The measurements were performed under different operating conditions of temperature and pressure, observing the effect of these variations on the ohmic, diffusion and activation losses of the cell. The experimental data were subsequently validated using a structural model that consists in construction of equivalent electrical circuits thatrepresent the electrochemical process of water electrolysis. The circuits are composed of concentrated elements, hence the already known electrical elements, and frequency-dependent elements, such as constant phase element (CPE) and Warburg element, that are useful for the representation of particular physical processes of the case study under consideration. The evaluation of the results as a function of temperature showed that the increase of the latter leads to a decrease of the charge transfer resistance of about 50% and 90% at the cathode and anode respectively. It influences the kinetics of the two evolution reactions, oxygen (OER), and hydrogen (HER), accelerating the processes with consequent lower activation losses. The increase in pressure, on the other hand, has a negative effect on the ohmic losses of the system, as it increases the ohmic resistance by about 30% at low temperatures, while at higher temperatures there is an increase of only 8%.