Lattice Boltzmann simulation of liquid water transport in gas diffusion layers of proton exchange membrane fuel cells: Parametric studies on capillary hysteresis

Water management is crucial for reliable operation of Polymer Electrolyte Membrane Fuel Cells (PEMFC). Here, the gas diffusion layer (GDL) plays an essential role as it has to ensure efficient water removal from and oxygen transport to the catalyst layer. In this study water transport through porous carbon felt GDLs was simulated using a 3D Color-Gradient Lattice Boltzmann model. Simulations were carried out on microstructures of plain and impregnated fiber substrates of a Freudenberg H14. The GDL microstructures were reconstructed from high-resolution X-ray micro-computed tomography ($\mu$-CT). For the distinction of carbon fibers and polytetrafluoroethylene (PTFE) in the binarized microstructures an in-house algorithm was developed. The additive was specified heterogeneously in the GDL through-plane direction employing a PTFE loading profile as derived based on $\mu$-CT image data. In the in-plane direction the additive was furthermore defined in a realistic fashion near carbon fiber intersections. Prior to parametric studies on capillary behavior a sophisticated modeling approach for semipermeable membranes had to be developed to account for experimental boundary conditions. Capillary hysteresis was then investigated by simulation of intrusion and drainage curves and subsequent comparison to testbench data

Exploring critical parameters of electrode fabrication in polymer electrolyte membrane fuel cells

Microstructure and electrochemical properties of the cathode catalyst layers (CCL) of a polymer electrolyte membrane fuel cells (PEMFC) have great impact on the performance and durability of the cell. The aim of this work is to establish a link between CCL structure and fuel cell performance. To obtain CCLs with unique structures six types of electrodes were prepared, each with a different coating technique but with the same Pt loading. The coating techniques are airbrush, screen printing, inkjet printing, dry spraying, doctor blade and drop casting. Moreover, intrinsic properties of PEMFC electrodes such as porosity, permeability, diffusivity as well as ionomer distribution are determined by Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) and Atomic Force Microscopy (AFM). Overall, 12 parameters have been evaluated. Generally, CCLs with low fractions of uncovered Pt/C show higher performance at low current densities. In this case the more homogeneous ionomer distribution leads to a higher catalyst utilization. At high current densities transport properties of the CCL have to be considered in addition to the catalyst utilization to explain their performance. The CCL prepared by screen printing shows a low fraction of uncovered Pt/C in combination with good transport properties, leading to the best performance at high currents

Lattice Boltzmann simulation of liquid water transport in gas diffusion layers of proton exchange membrane fuel cells: Parametric studies on capillary hysteresis

Water management is crucial for reliable operation of Polymer Electrolyte Membrane Fuel Cells (PEMFC). Here, the gas diffusion layer (GDL) plays an essential role as it has to ensure efficient water removal from and oxygen transport to the catalyst layer. In this study water transport through porous carbon felt GDLs was simulated using a 3D Color-Gradient Lattice Boltzmann model. Simulations were carried out on microstructures of plain and impregnated fiber substrates of a Freudenberg H14. The GDL microstructures were reconstructed from high-resolution X-ray micro-computed tomography ( -CT). For the distinction of carbon fibers and polytetrafluoroethylene (PTFE) in the binarized microstructures an in-house algorithm was developed. The additive was specified heterogeneously in the GDL through-plane direction employing a PTFE loading profile as derived based on -CT image data. In the in-plane direction the additive was furthermore defined in a realistic fashion near carbon fiber intersections. Prior to parametric studies on capillary behavior a sophisticated modeling approach for semipermeable membranes had to be developed to account for experimental boundary conditions. Capillary hysteresis was then investigated by simulation of intrusion and drainage curves and subsequent comparison to testbench data

Energy efficient cold start of a Polymer Electrolyte Membrane Fuel Cell coupled to a thermochemical metal hydride preheater

Cold start is still a major factor for proton exchange membrane (PEM) fuel cell degradation. Using a metal hydride-based preheater can significantly reduce the time to reach temperatures above 0 °C without consuming any extra energy due to two specific features of the fundamental thermochemical reaction: First, thermal energy provided during fuel cell operation as waste energy can be stored long-time and loss-free for the next cold start event. Second, providing a hydrogen pressure of 8 bar immediately triggers the exothermal absorption reaction to heat up a system from temperatures as low as −20 °C. The manuscript presents the first in time results of a system with a 1 kW PEM fuel cell starting from temperatures of −5 °C with and without an active preheating module at a hydrogen pressure of 6 bar. The experiments indicate that the single-cell voltage behavior is improved when the preheating module is active as the lowest values measured are in the range of 0.6 V in contrast to 0.45 V without a preheater. These findings are supported by simulation data of the modeled system, which indicates severe ice formation of up to 87 % in case of the cold start without a preheater in comparison to <1 % ice formation with a preheating module