Through-Plane Thermal Conductivity of PEMFC Porous Transport Layers

Abstract

We report the through-plane thermal conductivities of the several widely used carbon porous transport layers (PTLs) and their thermal contact resistance to an aluminum polarization plate. We report these values both for wet and dry samples and at different compaction pressures. We show that depending on the type of PTL and the existence of residual water, the thermal conductivity of the materials varies from 0.15 W K−1 m−1 to 1.6 W K−1 m−1, one order of magnitude. This behavior is the same for the contact resistance varying from 0.8 m2 K W−1 to 11×10−4 m2 K W−1. For dry PTLs, the thermal conductivity decreases with increasing polytetrafluorethylene (PTFE) content and increases with residual water. These effects are explained by the behavior of air, water, and PTFE in between the PTL fibers. It is also found that Toray papers of differing thickness exhibit different thermal conductivities.

Computation of Thermal Conductivity of Gas Diffusion Layers of PEM Fuel Cells

“…cycles of increasing compression up to a maximum value and releasing compression to 0 -was investigated and steady-state (in the investigated case reached after 5 cycles) properties were determined (Sadeghi et al, 2010). Further, it was found that residual water in the GDL leads to a significant increase of through-plane thermal conductivity (Burheim et al, 2010;Burheim et al, 2011). The influence of PTFE distribution on thermal conductivity is discussed in section 3.3.…”

“…Experimental results have shown that an increase of PTFE loading leads to a reduction of through-plane thermal conductivity (Khandelwal & Mench, 2006;Burheim et al, 2011) in several, but not all types of gas diffusion layers (see Table 2). 4.6 -13.9 bar compression dry GDL ex situ unidirectional heat flux and measuring temperature difference (Burheim et al, 2011) Toray carbon paper, Quintech, thickness 280 μm 0 0.33 -0.59 estimate based on experiment ex situ unidirectional heat flux and measuring temperature difference (Ramousse et al, 2008) 0.62 -0.89 4.6 -13.9 bar compression dry GDL 5 1.4 4.6 -13.9 bar compression residual water ex situ unidirectional heat flux and measuring temperature difference (Burheim et al, 2011) Assuming that the structure of the GDL remains unchanged, one would expect that the addition of material that has a higher thermal conductivity than air -like PTFE -should increase the overall thermal conductivity.…”

“…4.6 -13.9 bar compression dry GDL ex situ unidirectional heat flux and measuring temperature difference (Burheim et al, 2011) Toray carbon paper, Quintech, thickness 280 μm 0 0.33 -0.59 estimate based on experiment ex situ unidirectional heat flux and measuring temperature difference (Ramousse et al, 2008) 0.62 -0.89 4.6 -13.9 bar compression dry GDL 5 1.4 4.6 -13.9 bar compression residual water ex situ unidirectional heat flux and measuring temperature difference (Burheim et al, 2011) Assuming that the structure of the GDL remains unchanged, one would expect that the addition of material that has a higher thermal conductivity than air -like PTFE -should increase the overall thermal conductivity. However, since the thermal conductivity of PTFE is orders of magnitude smaller than that of carbon, PTFE potentially can insulate carbon fibers from each other, i.e.…”

“…Reversible as well as irreversible modifications of contact geometries -and consequently thermal conductivity could occur under mechanical loading. Also the observation that the presence of residual liquid water leads to an increase of through-plane thermal conductivity (Burheim et al, 2011) can possibly be explained; the water -with a thermal conductivity of 0.6 W/ m K (Taine & Petit, 1989) -could either improve heat flux at already existing contacts between fibers or even provide new heat flux paths that were 'insulated' by air before the addition of water.…”

“…Furthermore, a microporous layer (MPL, typical pore sizes around 100 nm) consisting of a mixture of carbon black and PTFE is often applied with a thickness of a few 10 µm on the side facing the catalyst layer for a further optimization of the water management (Paganin et al, 1996;Giorgi et al, 1998;Mathias et al, 2003). An operating PEM fuel cell is not isothermal, mainly because heat is generated within the membrane electrode assembly and at the same time this assembly can be considered 'insulated' by the gas diffusion layers (Burheim et al, 2011) leading to temperature gradients within the fuel cell. A detailed knowledge of the temperature distribution and therefore of thermal conductivity of the GDLs is essential for a proper understanding and the optimization of not only heat transfer in the PEM fuel cell, but also for water management and optimization of cell performance and durability.…”