An Analysis of the Influence of the Flow Field Channel Geometry of Stamped Metallic Bipolar Plates on the Performance of PEM Fuel Cell By Means of the Mathematical Modelling
Authors: Hala Miroslav , Kodým Roman , Prokop Martin , Paidar Martin , Bouzek Karel 
Published: September 2023

PEM fuel cells are a crucial link in the hydrogen economy because of their ability to transform the chemical energy of hydrogen into electrical energy with high efficiency at relatively low temperatures. Bipolar plates (BPs) are multifunctional components of PEM fuel cell stacks. Currently, material research in this field is focused on two promising material types: polymer carbon composites and metallic plates. Metallic plates have a number of advantages over composite materials, namely higher electrical conductivity, lower weight, and more effective manufacturing process. Typically, metallic BPs are manufactured with stamping or hydroforming. These methods are faster and cost-effective compared to machine milling or pressure extrusion typical for composite materials. Due to the nature of these processes, however, new channel geometries need to be considered. The first set of shape constrains comes from the forming properties of the material used. Other limitations result from the channel's ability to distribute the reagent gases, as well as to remove dissipate heat and water evolved. Traditional experimental testing methods involve an expensive and time-consuming production of samples and are unable to reliably determine local quantities. Within this study, mathematical modelling was used as a fast and cost-effective way of exploring the influence of the shape of the flow field channels on the distribution of local current density and species concentrations and performance of the PEM fuel cell.

A full 3D model accounting for the reactants flow dynamics, multicomponent mass transport, electric charge flow, and electrode reaction kinetics was proposed and implemented. The Butler-Volmer equation was used to model the reaction kinetics. A simpler 1D model was developed to obtain the necessary kinetic parameters through fitting experimental data. Stefan-Maxwell equations were used to simulate the multicomponent mass transport on the cathode side of the cell. The data required were collected from the literature [1]. Darcy’s law was used to describe the transport of hydrogen in the porous structure of the anode. Through means of parametrical variation of geometrical parameters, such as the channel ground and rib width, angle of the channel walls, and the radius of the corners, we were able to compare numerous geometries in terms of the performance of the whole fuel cell. Also, the distribution of the species and local current densities was obtained.

The width of the ribs and the ground of the channel were identified as the most influential parameters. In total, 18 combinations of channel ground and rib widths ranging from 0.1 to 4 mm and from 0.5 to 4 mm, respectively, were studied and compared to channels with square cross-section. In terms of the homogeneity of the local current densities, the narrowest combination offered the best results. This can be explained by the concentration of oxygen in the cathodic catalytic layer. In the case of wider channels, slow diffusion halted the transport of oxygen into the area of the catalytic layer at the closest distance from the channel ground, where the current was passing into the gas diffusion layer. This ultimately led to a large transport polarization and even fuel starvation in this area. Furthermore, in the well-supplied areas of the catalytic layer under the channels, where the bulk of gas transport is happening, local current densities were also low, because it is disadvantageous configuration for the electric charge flow. These two effects lead to a notably inhomogeneous local current density distribution with the most active area at the edge of the channel ground. In case of the narrow channels, it was possible to supply both reactants and charge into a larger area of the fuel cell, leading to much better utilization of the catalytical layer.

From the obtained results, qualitative intervals of the ideal channel geometry were proposed. The results obtained contribute to the efficient development and faster commercialization of novel BPs.

The CORE project Nr. TO01000306 benefits from Norway and Technology Agency of the Czech Republic Grant within the KAPPA Programme.

[1] Bird, R. B.; Stewart, W. E.; Lightfoot, E. N. Transport Phenomena, 2nd ed.; John Wiley & Sons, Inc.: USA, 2006.