Experimental and modelling investigation on degradation of the low temperature electrolyser through Electrochemical Impedance Spectroscopy

The objective of the present work is to investigate the performance of a single PEM electrolytic cell by carrying out electrochemical impedance spectroscopy measurements (EIS) coupled with polarization curves. Electrochemical impedance spectroscopy is a powerful non-destructive method to individuate the origins of the several polarization processes affecting the cell performance. In combination with other characterization methods such as polarization curves, EIS allows to distinguish several phenomena happening inside an electrolytic cell on the basis of the different relaxation times of each single process. Furthermore, it helps to quantify their impact on the efficiency and durability of the membrane electrode assembly (MEA). The methodology for the system’s characterization is based on two steps, the experimental procedure and the model development. In the first step several tests under different operating conditions are performed. Accordingly, the main variables identified are potential, current density, temperature and pressure. These variables which affect each process occurring within the cell differently, are varied for the tests. During operation at steady state condition, EIS tests are carried out and impedance data are collected. Subsequently, data quality check is needed to remove wild points and, data affected by inductive effects of cables and noise. Then, the equivalent circuit model (ECM) is chosen to simulate the impedance spectra trying to minimize the deviation between experimental results and fitting. The electrochemical parameters (double layer capacitance, polarization resistance, ohmic resistance) are estimated by complex non-linear least square (CNLS) analysis of the ECM model. To verify the validity of the results obtained from the impedance spectroscopy a comparison with the findings of the polarization curve is done. During the experiments, electrochemical characterization with polarization curves and EIS tests in potentiostatic and galvanostatic modes is performed on two MEAs with the same characteristics but different ageing conditions. Both potentiostatic and galvanostatic characterization methods have been performed at different temperature, pressure, cathode configuration and mass flow rate. Moreover, degradation tests are carried out to quantify the effects of the different phenomena on the performance of both cells. In conclusion, this work shows that electrochemical impedance spectroscopy along with polarization curves can provide useful information to individuate and quantify the contribution of the different processes affecting the performance of the electrolytic cell. Hence, it can help to find critical cell components that need to be improved in order to overcome performance and durability issues that currently create a barrier to the market penetration of this technology. In future works, a fundamental aspect to account for is addressing the ambiguity related to the equivalent circuit model which is used to simulate the experimental impedance spectrum. ECMs are generally chosen a priori, they do not describe the physicochemical properties of the system, but rather simply reproduction of the experimental data. Only a pre-knowledge of the system under study can guide on the choice of the exact ECM. Therefore, other methods such as distribution of relaxation times (DRT), can assist to the identification of a proper number of circuit elements avoiding problem of under- or over- fitting as well as lack of correlation with the physics.