Study of the relations between the performances of PEM fuel cell components and their behaviors in stacks operated in the complete system. : Development of electrical and mechanical characterization techniques

Low Temperature Polymer Electrolyte Membrane Fuel cell (PEMFC) is the most mature fuel cell technology, and it can be used in a variety of applications, (transport, space, stationary and portable applications). This electrochemical generator converts Hydrogen and Oxygen into electricity, heat and water through oxidoreduction reaction. PEMFC power production and overpotentials are related to the operating conditions and physical properties of cell components. Increasing cell efficiency can be achieved by reducing the overpotential losses, such as the Ohmic losses. This requires a better understanding of the relationship between the physical properties of the materials, their variation with regards to the operating conditions, and the influence of this variation on the Ohmic losses.A PEMFC is made of an electrolyte membrane covered with two catalyst layers, that are hold between two gas diffusion layers - GDLs and compressed by two Bipolar plates - BPPs. Ohmic overpotential losses are due to the protonic resistance of the membrane, to the electronic bulk resistance of the other components and to the contact resistance of interfaces between components. These resistances are function of the material nature and structure, the operating conditions such as humidity, temperature, and especially mechanical compression. This study contributes in understanding the origin of the electronic Ohmic losses, particularly those due to the GDL and its interfaces. The GDL plays several transport roles in the PEMFC by providing/evacuating reactant gases from/to BPP, conducting electrons and heat. It also contributes to water management and mechanically support (the catalyst coated membrane) so that it remains functional. The most common structure used to allow these various functions is a composite porous carbon fibers structure that can be paper or cloth. This porous structure is very sensitive to mechanical excitation which comes from variable external and internal sources generating unsteady states of stresses. In addition, the structure of the GDL exhibits a nonlinear compression stress-strain curve, with a strain hysteresis along the loading-unloading cycles. The physical properties of the GDL are affected by this behavior and need to be studied under cyclic compression in order to better approach the use conditions inside the FC, and to get a clearer idea about the contribution of the GDL to the global Ohmic losses. The optimal levels of mechanical compression must in particular be selected in order to make a trade-off between low Ohmic losses and high reactant diffusion rates.In this thesis, experimental investigations and analyses of electrical properties have been conducted on several types of commercial carbon paper GDLs:- Ex-situ characterization techniques have been used to measure the in-plane resistance according to two perpendicular directions. The impact of cyclic mechanical compression, the effects of temperature and humidity were investigated on the GDL through-plane resistance as well.- The electrical contact resistance between GDL and BPP was measured using the Transmission Line Method.It has been observed that all resistances decreased non-linearly with compression, meaning that optimal levels of compression can be obtained for PEMFC operation regarding the GDL porosity and diffusion properties. These resistances are more or less sensitive to the cycles of compression, according to the structure of the GDLs (the felt structure being the least sensitive towards cycles). The hysteresis of electrical resistance decreases with the rise of compression levels and with the number of compression cycles. Despite the GDL anisotropy observed for the in-plane resistance, some properties were unchanged with the measurement direction, such as the rate of resistance decrease with compression and the behavior towards the cycles of compression. Finally, in most cases, humidity was found to decrease the through-plane resistance of GDLs.