Statistical characterization of non-linear microscopic mechanical deformation through randomly oriented fibrous porous transport layers for advanced electrochemical energy systems

Abstract

A robust compression model based on the unit-cell beam bending theory is presented to predict the mechanical behavior of stochastically generated fibrous porous transport layers (PTLs) at various clamping pressures. For this purpose, three-dimensional PTLs were constructed by piling random paper type carbon-fiber structures. A representative elementary volume was determined based on the relative porosity gradient errors with a 95% confidence level for statistical analyses. Subsequently, a mechanical compression model based on the beam bending theory was developed to determine the microscale deformation characteristics of the PTLs for electrochemical energy systems. Based on the beam bending theory, carbon fibers are modeled as beams, and the bending of fibers is considered to be the main contributor to deformation of the PTLs. The numerical model shows good agreement with published experimental data in literature, i.e., a nonlinear stress-strain relationship. Next, the model was applied to feature bulk and local mechanical variations of the PTLs as functions of the number of carbon-fiber layers, porosity, polytetrafluoroethylene (PTFE) loading, and external clamping pressure. It was found that the addition of binder/PTFE into fibrous substrates results in the decreased porosity and increased mechanical strength of the PTLs. The detailed three-dimensional microscale deformation simulations revealed that the statistical mean strain of the PTLs was exponentially proportional to the porosity in the range 0.7-0.9 and decreased on addition of PTFE in the fibrous carbon substrate at a stack clamping pressure of 1 MPa. Moreover, the statistically estimated local strain distribution along the in-and through-planes of the PTLs indicated that the microscopic local deformation was approximately uniform through the PTLs. This modeling study can be utilized to understand the mechanical behavior of heterogeneous PTLs during external compression for advanced electrochemical systems.