A Methodology to Design the Flow Field of PEM Fuel Cells

Proton Exchange Fuel Cells (PEMFCs) are considered one of the most prominent technologies to decarbonize the transportation sector, with emphasis on long-haul/long-range trucks, off-highway, maritime and railway. The flow field of reactants is dictated by the layout of machined channels in the bipolar plates, and several established designs (e.g., parallel channels, single/multi-pass serpentine) coexist both in research and industry. In this context, the flow behavior at cathode embodies multiple complexities, namely an accurate control of the inlet/outlet humidity for optimal membrane hydration, pressure losses, water removal at high current density, and the limitation of laminar regime. However, a robust methodology is missing to compare and quantify such aspects among the candidate designs, resulting in a variety of configurations in use with no justification of the specific choice. This contrasts with the large operational differences, especially regarding the pressure loss/stoichiometric factor trade-off and in the outlet humidity level. In this paper a simple thermodynamic model (0D) is presented to evaluate pressure losses, stoichiometric factors, channel length, and humidity level for typical flow fields. Based on distributed and concentrated pressure losses and on a water balance between the humidified air, the electrochemically produced water, and the electro-osmotic water flux, the model indicates the optimal flow field for a given active area. The methodology is validated using 3D-CFD models, assessing the predictive capability of the simplified 0D model, and it is applied to small/medium/large active area cases. The presented method introduces a model-based guideline for the design of PEMFCs flow fields, providing design indications to optimize the humid flow dynamics. The study shows the impact of flow field design on fuel cell operating conditions, providing guidelines for fuel cell engineering. In the limits of laminar flows, the parallel channel design demonstrated the lowest pressure drop (1 × 102 - 103 Pa, more than one order of magnitude lower than other designs) and the best capability of saturated outlet flows (i.e., ideal membrane hydration) for current densities in the range 0.5 - 2.0 A/cm2, hence outperforming any other serpentine-type designs for medium-to-large active areas and with the focus on high current density operation.