(Invited) Review: PEMFC Materials' Thermal Conductivity and Influence on Internal Temperature Profiles
Authors: Burheim Odne Stokke 
Published: February 2020

The polymer electrolyte membrane fuel cell (PEMFC) is far from isothermal when in operation. This is well established both from experiments and from mathematical modelling. The latter relies on knowledge of heat transport and local heat generation in the different materials and cell design that constitute the PEMFC. Over the last 15 years the knowledge about thermal conductivity and heat generation in the PEMFC materials as a research field has been established and developed. From first looking at through-plane thermal conductivity of only membranes and gas diffusion layers (GDL), the most recent studies include heat transport dependency on non-isotropic properties, PTFE content, water content, compression, heat pipe effects, and material integration.  Depending on the fabric, dry GDL thermal conductivity range from 0.15 to 0.9 W K-1 m-1 when compressed at close to 10 bar compaction pressure. The most common fabrics have thermal conductivity between 0.3 and 0.4 W K-1 m-1. Adding water increases the thermal conductivity by between a factor of two and three, up to 1.5 W K-1 m-1, see ref 1. The in-plane thermal conductivity is larger and more than 10 W K-1 m-1, see ref 2-3. Thermal conductivity of fully humidified Nafion is 0.27 W K-1 m-1, see ref 4. Over the past few years, more attention has been given to the thermal conductivity of the microporous layer (MPL) and the catalyst layer (CL) and the way the MPL interact with the GDL, see ref 5-6. Thermal conductivity of these thin layers (CL and MPL) have been measured to be in the range of 0.1 W K-1 m-1, and in the region where the MPL and the GDL interfere to make a composite material, where the thermal conductivity has been estimated to be as high as 10 W K-1 m-1
. These different values and their historical development leads to a natural selection of 5 reference thermal models at 1 A cm-2 and 0.67 V, leading to temperature profiles under the rib of the current collector, shown i the attached figure. The base case consists of dry materials and no consideration of thin layer effects (CL and MPL), whereas the thin layer effects leads to additional temperature elevation. Accounting for the MPL-GDL interferences, on the other hand leads to a lowered temperature elevation inside the PEMFC. Adding water and accounting for these effects, and in addition considering the GDL with the highest thermal conductivity (wet Toray), the maximum temperature elevation under the polarisation plate land is lowered by a factor of 6 compared the base case of the oldest available thermal conductivity values. The attached figure shows the temperature profiles under the polarisation plate land for these five models and also indicate the regions; GDL, GDL-MPL-comp., MPL, Anode CL, Membrane, Cathode CL, MPL, GDL-MPL-Comp., GDL and the thickness, given by distance from the membrane. A richer and more detailed overview of thermal conductivity of several different materials and their effects on modelling thermal gradients inside the PEMFC is presented in the presentation and following paper at this conference. The take home message is that investigating thermal conductivity of PEMFC components is important for modelling and certainly not a closed chapter. References

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Figure 1