Lattice Boltzmann simulation of liquid water transport in gas diffusion layers of proton exchange membrane fuel cells

Abstract

Polymer electrolyte membrane fuel cells (PEMFCs) offer a compelling powertrain solution for the e-mobility sector and in particular for heavy-duty applications. Generating electrical energy by electrochemical reaction of hydrogen and oxygen to water, these fuel cells present (when using green hydrogen) a climate-friendly technology in support of reducing overall greenhouse gas emissions. During cell operation, the reactand gases are consumed at the electrodes and water is produced in the cathode catalyst layer (CCL). In dependence of the operating conditions, this product water can condensate and block as a liquid phase available transport paths for the reactand gases. In order to prevent advancing liquid water accumulation and therewith flooding of the cell, the product water has to be therefore removed efficiently. Thus, stable fuel cell (FC) operation is only possible with an appropriate water management. In this context, the porous gas diffusion layer (GDL) plays a pivotal role by ensuring both homogeneous reactand gas distribution to the electrodes and efficient water removal from the cathode catalyst layer. Consequently, optimal water management is only achievable with an appropriate design of the GDL material properties, which requires a profound understanding of the capillary transport phenomena on the pore scale. In long-term operation, the GDL is furthermore in general subject to different aging processes, due to the harsh conditions typically present in automotive applications. With proceeding material degradation, the water removal capabilities of the GDL can decrease over time, leading to a progressively deteriorated gas transport and increasing cell performance losses upon aging. In order to achieve the component durability needed to meet the lifetime requirements for long-term fuel cell operation, degradation phenomena and their repercussions on the water management therefore have to be fully understood. Owing to the complexity of multiphase transport mechanisms in porous media, however, conventional experimental testing can be expensive, time-consuming and with limited depth of detail. In recent years, pore-scale modeling (PSM) has therefore gained increasing popularity as a comparatively fast and inexpensive technique for investigation of porous media transport processes directly at the pore scale. In this work, liquid water transport through a carbon felt GDL was thoroughly investigated using multiphase PSM simulations and real microstructural data. At first, a 3D Color-Gradient Lattice-Boltzmann model was developed and validated against analytical references. GDL microstructures of a plain and an impregnated carbon fiber substrate of a Freudenberg H14 GDL were then reconstructed via segmentation of high-resolution X-ray micro-computed tomography (µCT) images. For the microstructure reconstruction of the impregnated GDL, an in-house algorithm was furthermore developed to distinguish a hydrophobic additive component (polytetrafluoroethylene (PTFE)) from the support material (carbon fibers). Subsequently, a first computational domain for the simulation of GDL liquid water transport was generated according to the experimental boundary conditions of a test bench for measuring capillary pressure-saturation (pc - S) relations. Here, special attention was paid to the GDL surface regions by explicit consideration of two semipermeable membranes according to the experimental setup. Starting from an initially dry GDL, liquid water intrusion was then simulated by gradually decreasing the gas pressure until full saturation was reached. Whereas the obtained pc - S relation is not unique but dependent on the wetting history of the GDL, drainage of liquid water was simulated as well, thereby recovering the complete capillary hysteresis. Parametric studies then showed that the obtained pc - S characteristics are significantly affected by boundary effects owing to the highly porous GDL surface regions. In addition, microstructure-resolved simulations require a high resolution of the porous medium in order to predict capillary behavior reasonably. Assuming uniform wettability with a carbon fiber contact angle of θ_CF = 65° for the plain fiber substrate, the simulated pc - S curves were furthermore in good agreement with the experimental reference. Considering mixed wettability for the impregnated fiber substrate, on the other hand, the simulations showed an expected shift of the pc - S characteristics towards higher capillary pressures owing to the hydrophobicity of the additive (PTFE) but could otherwise not reproduce the experimentally observed significant enlargement of the capillary hysteresis. This discrepancy was primarily related to a measurement artifact. After validation of the simulated capillary hystereses, novel pc - S relations were furthermore derived for both the intrusion and drainage of liquid water in the plain and impregnated H14 fiber substrate. On the other hand, the widely-used Leverett relation was found to be incapable to describe the capillary characteristics of the plain and impregnated carbon felt GDL appropriately. In order to investigate the impact of aging on the GDL water management, a second computational domain was generated, mimicking the operando conditions during fuel cell operation. For this purpose, the GDL microstructure reconstruction was virtually compressed corresponding to a typical cell assembly clamping pressure. In addition, a microporous layer (MPL) was considered as well by reconstruction of its macropores via segmentation of µCT images of an impregnated and MPL-coated Freudenberg H14 GDL. Generating a computational setup, the GDL/MPL assembly was then sandwiched between a liquid and a gas buffer zone representing the catalyst layer (CL) and the gas channel (GC) according to operando conditions. In order to investigate aging effects, the microstructure reconstructions were furthermore degraded virtually assuming PTFE loss from the GDL and increase of the MPL macroporosity as the main aging scenarios. Corresponding to stationary cell operation, liquid water transport through the partially aged GDL and MPL was then simulated with a constant inlet velocity. Once the liquid phase percolated through to the gas channel (i.e., at breakthrough), the simulations were halted and the invasion patterns were then characterized by means of local and overall saturation. In order to investigate mass transport limitations due to liquid water accumulation in the pore space, effective gas transport properties were furthermore determined for the partially saturated and aged GDLs and MPLs. Subsequent simulations then showed that with the MPL in the pristine state loss of PTFE had no measureable effect on the liquid water transport through the degraded GDL. This observation was related to the dominant impact of the MPL on the capillary transport by strongly restricting the liquid water invasion sites into the GDL. Upon aging of the MPL, on the other hand, significant degradation effects were observed, as indicated by rising breakthrough saturations and an increasingly disturbed gas transport. Once the MPL was already degraded, aging of the GDL was then found to impact the liquid water transport and consequently the effective gas transport as well.