Degradation root cause analysis of PEM fuel cells using distribution of relaxation times

Proton exchange membrane (PEM) fuel cells use hydrogen as fuel and produce only water and heat as byproducts. This makes them a promising technology for the worldwide energy transition. However, durability and efficiency remain two main obstacles to their widespread commercialization. Degradation of fuel cells is thus a key focus in recent studies. The majority of existing works use voltage or power as the global health indicator for evaluating the degradation of a running fuel cell. The use of global health indicators is limited in distinguishing the contributions of individual physicochemical degradation processes to the overall performance degradation. This work proposes to investigate the degradation root cause of PEM fuel cells using the distribution of relaxation times (DRT) analysis. DRT is a model-free approach that can separate the overlapped polarization processes by transforming the frequency-domain impedance data into distribution of time constants. Datasets from two long-term durability tests are used to calculate the DRT and identify the degradation root causes. Four distinct peaks are identified from the electrochemical impedance spectroscopy spectra, which correspond to the mass transport, charge transfer of the oxygen reduction reaction, proton transfer, and interface contact. The results show that the degradation of the fuel cell stacks is mainly caused by the degradation of the cathode catalyst layer and the gas diffusion layer. Specifically, these two processes account for 91.2% and 90.4% of the overall degradation in FC1 and FC2, respectively. Moreover, it is found that the mass transport resistance contributes a larger percentage under the dynamic current load (38.2%) compared to the constant load (23.1%). Thus, the oxygen diffusion process needs to be enhanced by proper operating strategy or improving the design of the cathode catalyst layer and gas diffusion layer for dynamic load applications. The proposed method provides a new perspective for the degradation analysis and prognostics of fuel cells, which can be further used for the development of efficient control and management strategies to ensure enhanced durability of fuel cells. • PEM fuel cell degradation analysis based on impedance spectra. • The polarization processes are quantified by distribution of relaxation times. • Degradation root cause identified by contributions of overpotential losses. • Two long-term durability tests are used to validate the degradation analysis strategy.