Reactant Gas Transport in PEM Fuel Cells

Power losses associated with slow reaction kinetics and mass-transport limits in PEM fuel cells can be strongly influenced by convective flow characteristics. Specifically, convection in the form of channel bypass may be utilized to simultaneously increase reactant concentration and reduce product concentration in the catalyst layer, thus reducing the activation and mass-transport overpotentials. We have developed an analytical model to predict the flow pattern and pressure field in general single-serpentine flow field geometries.

Using the equations of continuity, viscous losses in channel flow, and Darcy flow for convective bypass, it is possible to solve for the pressure along a given serpentine channel as:



Then the mean velocity along the channel is:



and the convective bypass through the GDL is:



Here,



represents a grouping of channel dimensions, GDL thickness, and permeability.

The model predicts that a significant portion of the total flow through the fuel cell can occur as in-plane convective flow through the gas diffusion layer under realistic operating conditions. The relative influence of convection depends highly on in-plane permeability of the gas diffusion layer and channel length, and is relatively independent of gas diffusion layer thickness. The Peclet number is used to quantify the relative importance of convection over diffusion:



A Peclet number plot shown below indicates that for small cell, Pe < 1, meaning that diffusion dominates. But for larger sized cell, Pe > 1, and convection dominates.



By designing fuel cells to utilize enhanced in-plane convection, it is suggested that losses associated with low oxygen content as well as liquid water buildup in the catalyst layer can be reduced.