Multiscale structural-fluidic coupled approaches to mass transport enhancement in deformable porous transport media for fuel cell systems

Abstract

The purpose of this study is to analyze the mechanical behavior of deformable porous transport media (PTM) under stack clamping pressure in fuel cell systems and the effects of non-uniform deformation on internal mass transport characteristics and overall performance in terms of structural analysis, fluid dynamics, and electrochemical reactions. Structural-fluidic approaches to optimizing gas flow channels stamped on sheet metal with various design parameters are implemented to identify the fluid–structure interaction characteristics of locally deformed PTM and the entropy generation rate for bypass flow through the PTM. First, static structural analysis is conducted to demonstrate PTM deformation by stack compression and its mechanical effects on the fluidic properties of PTM. The PTM-channel model results agree with the experimental results within a maximum error of less than 10%. Emphasis is placed on describing how the flow of reactant gas through the PTM effectively transports the oxygen gas to the catalyst layers. Next, parametric studies are conducted to determine the dominant design effects on fluidic performance over the entire computational domain. The main design parameters for the cross-section of the gas channel are selected: channel-to-rib width ratio, draft angle, inner fillet radius, and channel depth. The flow pattern is analyzed by quantifying various mass transport parameters (i.e., pressure drop, bypass flow ratio, oxygen transport ratio, and average entropy). Subsequently, a design optimization method is applied to obtain the most favorable flow channel designs with trapezoidal cross-sections. In the optimized serpentine channel design, the maximized oxygen transport ratio is predicted to be 0.718 at the interface between the PTM and the catalyst layers under the constraint of total drop in pressure. The analysis of the mechanical deformation of PTM is then extended to the micro-scale in order to investigate the internal non-uniform mechanical behavior. Micro-macro structural analysis facilitates the active investigation of the mechanical characteristics of PTM and the characteristics of internal mass transport. First, random structures with a representative elementary volume (REV) are generated with carbon fiber arrangement. After the statistical validation of REV selection, static structural analysis on the microstructure model is performed to obtain the locally variant mechanical behavior and corresponding effective mechanical properties of the PTM. With a precisely corrected strain-stress rate function in the through-plane direction, macro-scale structural analysis is conducted to determine the geometrical interaction between the bipolar plate channels and the PTM. Finally, conjugated structural-fluidic models with electrochemical reactions are developed to compare the mass transport rates and corresponding fuel cell performance before and after compression. These modeling results enable a priori insight to be gained into the morphological effects of porous media with carbon fibers on mechanical characteristics and electrochemical reactions in fuel cell systems. In the final part, a numerical investigation of anisotropic fluidic properties of PTM is conducted. Three-dimensional electrochemical reaction analysis is employed to identify the effect of variations in porosity and permeability on fuel cell performance. Mass velocity and oxygen concentration distribution at the interface between PTM and the catalyst layer are described for a wide range of gas permeabilities in the PTM and the porous bipolar plate flow-fields. Permeability in the porous media is varied in both the in-plane and through- plane directions, and the local current density distribution and polarization curves are analyzed for the concentration voltage loss. These results can be used in the development of advanced stack component designs for enhanced fuel cell systems.