Bulk and interfacial thermal transport in microstructural porous materials with application to fuel cells

The performance, reliability and durability of fuel cells are strongly influenced by the operating conditions, especially temperature and compression. Adequate thermal and water management of fuel cells requires knowledge of the thermal bulk and interfacial resistances of all involved components. The porous, brittle and anisotropic nature of most fuel cell components, together with the micro/nano-sized structures, has made it challenging to study their transport properties and thermal behavior. The main purpose of this research was to explore, and guide the improvement of, the thermal behavior of fuel cell materials under compression. Thickness-based methods, having the capability of deconvoluting bulk from the contact resistance, were employed to accurately measure the thermal conductivity of several gas diffusion layers (GDLs) with different PTFE loading. The interfacial thermal resistances of these GDLs with adjacent micro porous layer (MPL) and graphite bipolar plate (BPP) were also determined, through both systematic experiments and comprehensive models developed in this work. The thermal conductivity of a coated MPL as a function of compression and that of a Ballard graphite BPP with respect to temperature were also measured and reported in this thesis. Higher values of contact resistance compared to the bulk resistance at low compression and the reduction of GDL thermal conductivity with PTFE loading are among the main findings of this study. The present work also revealed the following novel counter-intuitive facts: (i) contact resistance may decrease with increasing the porosity of the mating porous materials; (ii) the conventional notion that the thermal conductivity of fibrous materials decreases with increasing porosity does not necessarily hold; and (iii) fiber spacing can be as crucial as porosity to the transport properties of fibrous media. The main conclusion is that the equations that are based solely on porosity should be either discarded or used, with caution, over the limited range of conditions under which they have been formulated. Through a series of experiments combined with theoretical analyses, this thesis presents some key data that helps unravel some unexplained trends reported in the literature. It also provides novel insights into the unexplored thermal behavior of fuel cell components and guides the modification of their micro-structures for better heat management of fuel cells.