Modeling Proton Exchange Membrane Fuel Cell Cathode Catalyst Layers with the Lattice-Boltzmann-Method Framework

Abstract

In this work, a coupled Lattice-Boltzmann-Method and electrochemistry Direct-Numerical-Simulation model for a proton exchange membrane (PEMFC) electrode has been presented. One of the main challenges affecting study of the cathode catalyst layer (CCL) in PEMFCs is the lack of detailed understanding of species transport and how it affects electrochemical performance. Researchers have typically used high level approximations that oversimplify the microstructure of the CCL—these are known as macrohomogenous models. However, as the field has progressed, these idealizations have begun to show their flaws, especially in areas of improving catalytic performance with lower Pt-loadings and non-noble metal catalysts. Previously, the microstructure details needed to build an accurate mesoscale model have eluded researchers; however, with advances in tomography and focused-ion-beam scanning-electron-microscopy (FIB-SEM), creating these representations has become possible. Mesoscale modeling in the CCL has been traditionally approached through either the Lattice-Boltzmann-Method (LBM) or electrochemistry coupled Direct-Numerical-Simulation (DNS). These models have been underutilized in the fuel cell community due to their complexity and resource intensiveness; however, with advances in parallel computing, this has become not only a possibility, but a necessity for modeling phenomena such as low platinum loadings and interfacial effects. With these new advances, a synergistic modeling approach can be taken that combines the advantages of each method. This can shed light on the transport and degradation phenomena in PEMFCs, particularly catalyst layer considerations and carbon support corrosion. Here, the liquid water as well as gaseous oxygen, nitrogen, and water vapor profiles through segmented geometries made of tomography measurements from FIB-SEM were shown through LBM. This is shown in Figure 1, where the capillary pressure is varied in six stages from 1.8 MPa to 20 MPa, which shows a saturation increase from 5% to 60%, respectively. This diagram shows the capillary fingering regime we should expect through the porous media. Additionally, these same geometries were used to simulate electrochemical phenomena, such as reaction profiles, polarization curves, ionomer thickness, and many others. Additionally, to examine the effects of ionomer coverage on different interfaces, a physics-based model using erosion was used to add in the ionomer, as FIB-SEM is unable to differentiate between ionomer, Pt, or carbon support. Once in, on the LBM side, we can model the ionomer by changing its contact angle compared to the carbon support (hydrophilic or around 10° for the ionomer compared to hydrophobic or 110° to 140° for the support), whereas for DNS, it is taken into account by considering Henry’s law and other transport phenomena. Finally, to account for accurate liquid formation throughout the geometry according to the Pt distribution, a source term was included in the LBM model and then transferred to DNS to create one of the first numerically intensive frameworks for a PEMFC electrode available. Figure 1