A fuel cell stack consists of hundreds of cells in series. Because the current passes through all of them, the lowest-performing cell limits the whole stack — and that cell is usually the one the reactant reaches least.
A distribution problem on two scales
Stack scale. Distribution from the main supply manifold to the cells. If the pressure falls along the manifold, the cells at the end are underfed. The manifold cross-section being large enough relative to the cell channels is therefore critical.
Cell scale. Distribution within the channel pattern of a single plate. In parallel channels water accumulation can block some channels and divert the flow to others; a serpentine pattern prevents that but has a high pressure loss. Intermediate (interdigitated) patterns force flow through the gas diffusion layer.
Both are classic distribution problems.
Water management is the central issue
In proton exchange membrane cells the membrane has to stay humid to conduct; but if too much water is produced at the cathode it blocks the pores (flooding) and prevents the reactant from reaching the catalyst.
The target is therefore not to dry but to stay within a narrow humidity band, and that band varies with current density, temperature and position. One channel can be dry while its neighbour is flooded.
Water transport takes place within the porous layers, and the porous media properties (porosity, permeability, wettability) set the result directly.
The thermal side
Part of the efficiency turns into heat, and the membrane temperature has to be held in a narrow band. The cooling channels are inside the plate, and the temperature distribution should also be equal from cell to cell. The problem is conjugate.
In modelling, flow, species transport, the water phase and heat are solved together; looking at the flow distribution alone gives an incomplete picture.
References
- Ergun, S. (1952). Fluid flow through packed columns. Chemical Engineering Progress, 48(2), 89–94.
- Shah, R. K. & Sekulić, D. P. (2003). Fundamentals of Heat Exchanger Design. John Wiley & Sons. DOI: 10.1002/9780470172605