A fully parameterized microscale model for lithium ion cells is presented in which the solid and pores (filled by electrolyte) are spatially resolved, and the mass and charge transport equations describing diffusion and migration in each phase are solved separately. Such a model allows: (1) the correlation of structure-scale, non-homogeneous material properties with macroscopic battery performance, and (2) the correlation of geometrical electrode morphology with macroscopic battery performance (electrode design). The micro-model approach discussed here allows for a simpler parameterization as fewer constitutive relations are needed in contrast to the macro-homogenous physical-based approaches. Input parameters were measured experimentally on lithium manganese oxide electrodes and LiPF 6 in 3:7 EC:DMC. Verification and validation for the model is also reported.

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For better water management in gas channels (GCs) of polymer electrolyte fuel cells (PEFCs), a profound understanding of the liquid water dynamics is needed. In this study, we propose a novel geometrical setup to conduct a series of direct simulations of the liquid water dynamics in a GC. The conducting pathways in the gas diffusion layer (GDL) are simplified by three cylindrical pipes connected to a liquid water reservoir representing the catalyst layer (CL). The droplet dynamics, corner film dynamics, and the competition between the film and droplet flows in the GC are explored in detail. The results show that the three-phase contact line plays an important role in resisting the gas drag force for a droplet movement in the GC. The gas drag force can dominate the film flow along the GC corners, and a proper selection of the contact angle of the GC sidewalls is necessary to balance two requirements: increasing the film removal ability and removing the water clogging fast. The competing mechanisms of the droplet and film flows give us the possibility to regulate liquid water flow into GCs, and maybe lead to a better water management in GCs. Finally, the results from this work also serve to provide insights into the development of a phenomenological model for the liquid water flooding in GCs.

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In a polymer electrolyte membrane (PEM) fuel cell water is produced by electrochemical reactions in the catalyst layer on the cathode side. The water diffuses through the catalyst layer and a fibrous substrate into gas channels where it is transported away by convection. The fibrous substrate represents the gas diffusion media (GDM). Sometimes the GDM has a thin microporous layer on the side facing the catalyst layer. The same layer structure can be found on the anode side. All layers together are the porous layers of a PEM fuel cell. Under certain operating conditions condensation can occur in the porous layers which might lead to flooding conditions and — if the liquid water forms droplets which grow together in the gas channels — the complete blockage of the channels. Both situations can lead to a local starvation of reactant gases with negative impact on fuel cell performance and durability. The void space of the hydrophobic fibrous substrate in a PEM fuel cell can be interpreted as micro channels in a broader sense, especially if liquid phase transport from the catalyst layer towards the gas channels is in focus. Due to the small dimensions with effective channel diameter in the range of micrometer the flow of liquid water is governed by capillary forces. The same applies for the gas channels at low gas velocities since the Bond and Capillary numbers are well below one. Thus the investigation of liquid water flow and distribution under low gas velocities in the hydrophobic fibrous substrate and the spreading of liquid water along the hydrophilic gas channel walls under capillary action is of special interest for PEM fuel cells and investigated here.

The mathematical description of liquid water flooding in the gas channel (GC) of a polymer electrolyte fuel cell (PEFC) at the macro scale has remained a challenge up to now. The mist flow assumption in the GC has been commonly used in previous numerical studies. In this work, a one-dimensional (down-the-channel) macroscale phenomenological model for the liquid water flooding in the cathode GC is developed based on several reasonable assumptions. We focus on the operating conditions with fully humidified inlet air on the cathode side. Some simplifications are introduced to obtain a manageable numerical model. A series of case studies are conducted to investigate the effects of droplets and various operating parameters on the liquid water flooding in the GC. The results show that the gas drag force at the film-gas interface significantly enhances the film flow along the GC corners. Based on a proposed droplet model, we can capture key scenarios of liquid water flooding in the GC under different operating conditions like varying stoichiometry, current density and cell temperature. We find that droplets in the GC mainly contribute to the gas pressure loss and water flooding in the GC, which should be depressed for better water management. At last, a proper selection of the GC sidewall contact angle is found to be critical to a better water management in the GC.

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