Behavioural analysis of PEM fuel cell components and research of causal relationships between in-situ and ex-situ observed performances

Polymer Electrolyte Fuel Cells (PEFCs) have shown their potential for both mobile and stationary applications. Assembling the PEFC stacks requires an appropriate mechanical compression in order to achieve adequate contacts (i.e. mechanical, thermal, and electrical) between the fuel cell components and to ensure gas-tight operation. However, an excessive mechanical compression may worsen the fuel cell performance as a result of the reduction in the porosity and mass transport ability of the Gas Diffusion Layers (GDLs). The work presented in this thesis aims to investigate the effects of 12 levels of mechanical compression, ranging from 0.35 to 2 MPa, on the in-situ performance of a 225 cm2 PEFC. The experimental investigations present the novelty of combining various characterisation techniques (i.e. polarisation measurement; Electrochemical Impedance Spectroscopy - EIS; local current/temperature and pressure distributions analysis; displacement measurement) while providing correlations with ex-situ results obtained within the MIREPOIx research project.The results show that a mechanical compression up to 1.55 MPa improves the PEFC performance at all tested operating conditions. This finding was attributed to the dominant reduction of the ohmic resistance against the increase in mass transport resistance. The results also emphasise the fact that mechanical compression up to 1.55 MPa improves the distributions of current and temperature. This effect was attributed to the enhanced pressure distribution achieved with increasing mechanical compression. The results obtained in this thesis suggest that compressing the PEFC beyond a specified level (1.55 MPa) would not lead to any further improvement of the PEFC performance. It may even worsen this performance due to the increase in the mass transport resistance. Finally, valuable correlations between the results from in-situ and ex-situ characterisation techniques are provided and analysed throughout the thesis.