On continuum models for heat transfer in micro/nano-scale porous structures relevant for fuel cells

Micro or even nano-scale solid particles are applied in porous structures in different energy systems, such as fuel cells, for the objectives to enhance the catalytic reaction activities and improve the fuel utilization efficiency or/and reduce the pollutants. In addition to the charge transport and reactions, heat transfer processes in fuel cell porous electrodes are strongly affected by the small scale and complex porous structures. In this paper, the thermal energy equations commonly used for continuum models at pore-level and porous-average level are outlined and highlighted, with the purpose to provide a general overview of the validity and the limitation of these approaches. Various models in the open literature are reviewed and discussed focusing on the important properties in the continuum methods, e.g., the effective thermal conductivity and interfacial/volumetric heat transfer coefficient between the fluid and solid surfaces. It is revealed that both the rarefaction and tortuous effects may be significant, but these have not been extensively studied yet in the micro/nano-scale heat transfer models relevant for the fuel cells. Comments and suggestions are presented for better understanding and implementation of the continuum heat transfer models for fuel cell electrodes.