Abstract

High temperature polymer electrolyte membrane fuel cells generate electricity from hydrogen and, for example, may be used as an alternative to polluting diesel generators in stationary power generation applications. This study focuses on the bipolar plates (BPs) which are critical for ensuring, e.g., proper electrical performance of the fuel cell. However, BPs are responsible for a significant portion of the fuel cell stack cost. Thus, to reduce expenses, a thorough understanding of how the materials making up the BPs influence its properties is required. Specifically, this PhD thesis will focus on the electrically conductive fillers commonly used in polymer composite BPs by studying a model system comprising said fillers in a Newtonian mineral oil.

First, suspensions of the main filler, graphite, are studied. Steady shear rheometry reveals the suspensions shear thin and exhibit a yield stress. The results are successfully described by a recent constraint-model, suggesting the rheology is controlled by frictional and adhesive contacts.

Next, graphite is dispersed in a colloidal gel of carbon black. These binary composites are shown to exhibit a strong flow-history dependence with shear acting as a ‘switch’ between a high-yield-stress, high-conductivity state and a low-yield-stress, low-conductivity state. Electron microscopy reveals the former is associated with a homogeneous dispersion of fillers while the latter microstructure shows phase separation of carbon black into sparsely connected blobs. A similar microstructural transition is observed when substituting conductive graphite for insulating glass spheres, except the elasticity and conductivity of such composites now become decoupled.

In summary, this research enhances our understanding of how shear flow may control the structure and properties of similar suspensions.