Reactant Gas transport in
PEM Fuel Cells
Power losses associated
with slow reaction kinetics and mass-transport limits in PEM fuel
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:
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
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.