(Invited) Thermal Gradients and Thermal Conductivity in PEM Fuel Cells, Compared to Li-ion Batteries and Super Capacitors
Authors: Burheim Odne Stokke , Pollet Bruno G 
Published: February 2020

The PEM fuel cell has been developed into a very effective energy converter. That is, the development efforts on PEM fuel cell has been mainly focussed in terms of output power density, rather than in terms of energy efficiency; the operational cell voltage remains constant and the current density increases. In turns the heat flux, released from the reaction increases too [1]. This heat flux leads to increased temperature gradients inside the PEMFC, and in particular across the gas diffusion layer (GDL), the microporous layer (MPL) and the catalyst layers (CL). Over the past decade, several studies of the thermal conductivity of PEMFC components have been undertaken in order to provide models with more accurate thermal conductivity data as a function of: compression, PTFE loading, ageing, water content, direction (in- vs. through-plane), material types, and material integration. As many of these now seem to be well understood, there are still room for more fundamental mechanistic studies, which are highlighted in this presentation. With a modern fuel cell operation at a cell voltage of +0.7V, one can easily see temperature differences between the polarisation plates and the membrane of more than 10oC, depending upon the state of water and material selection. [2] A summary of PEMFC material thermal conductivity and gradients is also described in this presentation.

Heat transfer in the components in a PEM fuel cell and the corresponding temperature gradients is dominated by heat flux through the passive components (GDL and MPL), leading to linear gradients. In other electrochemical energy storage devices such as supercapacitors (SC) and lithium ion batteries (LiB), it is the active materials (electrodes and separators) that dominate the temperature gradients, which in turn becomes more parabolic (2nd order). From an engineering point of view, it is interesting to compare thermal gradients and internal temperature differences of a PEMFC to these other devices, because; the thermal conductivity of the heat generating materials are similar for the three, the operation voltage is higher in a LiB and SC than in a PEMFC, the electrode thicknesses are larger in a LiB and SC than in a PEMFC; there are less opportunities for cooling in side a LiB and SC than in a PEMFC, and remarkably; relevant current density of a PEMFC, SC, and LiB are 30,000, 200, and 50 A m-2, respectively. [1,3,4] A brief comparison of PEMFC, SC, and LiB thermal conductivity and gradients are discussed in this presentation.

[1] O. S. Burheim, J. G. Pharoah, A review of the curious case of heat transport in polymer electrolyte fuel cells and the need for more characterisation, Current Opinion in Electrochemistry, (2017).

[2] O. S. Burheim, "Review: PEMFC Materials' Thermal Conductivity and Influence on Internal Temperature Profiles, ECS Transactions, 80 (2017) 509-525.

[3] F. Richter, S. Kjelstrup, P. J. S. Vie, O. S. Burheim, "Thermal conductivity and internal temperature profiles of Li-ion secondary batteries", Journal of Power Sources 359 (2017) 592-600.

[4] H. H. Hauge, V. Presser, O. Burheim, "In-situ and ex-situ measurements of thermal conductivity of supercapacitors", Energy, 78 (2014), 373-383