Coupled continuum and condensation-evaporation pore network model of the cathode in polymer-electrolyte fuel cell

A model of the cathode side of a Proton Exchange Membrane Fuel Cell coupling the transfers in the GDL with the phenomena taking place in the cathode catalyst layer and the protonic transport in the membrane is presented. This model combines the efficiency of pore network models to simulate the liquid water formation in the fibrous substrate of the gas diffusion layer (GDL) and the simplicity of a continuum approach in the micro-porous layer (MPL). The model allows simulating the liquid pattern inside the cathode GDL taking into account condensation and evaporation phenomena under the assumption that the water produced by the electro-chemical reactions enters the MPL in vapor form from the catalyst layer. Results show the importance of the coupling between the transfers within the various layers, especially when liquid water forms as the result of condensation in the region of the GDL fibrous substrate located below the rib

Water transport in gas diffusion layer of a polymer electrolyte fuel cell in the presence of a temperature gradient. Phase change effect

The gas diffusion layer (GDL) is a crucial component as regards the water management in proton exchange membrane fuel cells. The present work aims at discussing the mechanisms of water transport in GDL on the cathode side using pore network simulations. Various transport scenarios are considered from pure diffusive transport in gaseous phase to transport in liquid phase with or without liquid–vapor phase change. A somewhat novel aspect lies in the consideration of condensation and evaporation processes in the presence of a temperature gradient across the GDL. The effect of thermal gradient was overlooked in previous works based on pore network simulations. The temperature gradient notably leads to the possibility of condensation because of the existence of colder zones within the GDL. An algorithm is described to simulate the condensation process on a pore network

Pore network model of the cathode catalyst layer of proton exchange membrane fuel cells: Analysis of water management and electrical performance

A pore network modeling approach is developed to study multiphase transport phenomena inside a porous structure representative of the Cathode Catalyst Layer (CCL) of Proton Exchange Membrane Fuel Cell. A full coupling between two-phase transport, charge transport and heat transport is considered. The liquid water evaporation is also taken into account. The current density profile and the liquid water distribution and production are investigated to understand the liquid production mechanism inside the CCL. The results suggest that the wettability and the pore size distribution have an important impact on the water management inside the cathode catalyst layer and thus on the performances of the proton exchange membrane fuel cell. Simulations show also that Bruggemann correlation used in classical models does not predict correctly gas diffusion

Pore network simulation of water condensation in Gas Diffusion Layers of PEM Fuel Cells

The fundamental understanding of water transport in PEMFCs is still a major challenge in direct relation with the water management issue, i.e., the ability to maintain a good dynamic balance of water in the membrane-electrode assembly during operation. We concentrate on the water transfer mechanisms occurring in the gas diffusion layer (GDL) on the cathode side. Recent works, e.g. [1], suggest that water condensation plays a major role in the water transfer and the flooding of GDL. In the present work, condensation in the GDL is studied numerically from three dimensional pore network simulations. As illustrated in Fig.1, the simulations predict that water condenses under the rib, in qualitative agreement with experimental visualizations [2]. The condensation process leads to the formation of growing liquid clusters progressively invading the GDL from the rib. This mechanism of flooding by condensation is markedly different from other scenarios assuming flooding by liquid water from the active layer – GDL interface, e.g. [3]. This new pore network model opens up the route to determine the exact water invasion mechanisms (liquid invasion and/or condensation), which is crucial for improving both performance and durability of PEMFCs

Pore network modelling of condensation in gas diffusion layers of proton exchange membrane fuel cells

A pore network model (PNM) is exploited to simulate the liquid water formation by vapour condensation in the gas diffusion layer (GDL) on the cathode side considering the spatial temperature variations within the GDL. The computed distributions are markedly different from the ones computed in previous works assuming capillarity controlled invasion in liquid phase from the catalyst layer and found to be in quite good agreement with several experimental observations. The proposed model opens up new perspectives for understanding the water transfer in protons exchange membrane fuel cells and the associated water management and aging issues

Water condensation in Gas Diffusion Layers of PEM Fuel cells

The fundamental understanding of water transport in PEMFCs is still a major challenge in direct relation with the water management issue, i.e., the ability to maintain a good dynamic balance of water in the membrane-electrode assembly during operation. In the present effort we concentrate on the water transfer mechanisms occurring in the gas diffusion layer (GDL) on the cathode side. In – situ visualizations of liquid water in GDL [1] and evaluations of temperature variations across the GDL [2] suggest that water condensation plays a major role in the water transfer and the flooding of GDL. In this work, condensation in the GDL is studied numerically from three dimensional pore network simulations. As illustrated in Fig.1, the simulations predict that water condenses under the rib, in qualitative agreement with the experimental visualizations [1]. The condensation process leads to the formation of growing liquid clusters progressively invading the GDL from the rib. This mechanism of flooding by condensation is markedly different from other scenarios assuming flooding by liquid water from the active layer – GDL interface, e.g. [3]. This new pore network model opens up the route to determine the exact water invasion mechanisms, which is crucial for improving both performances and durability of PEMFCs

On the current distribution at the channel - rib scale in polymer-electrolyte fuel cells

Experimental results based on in-situ measurements at the interface between the catalyst layer and the gas diffusion layer (GDL) on the cathode side at the channel e rib scale show an interesting variation of the current density distribution as the mean current density is increased. It is found that the local current density below the rib median axis corresponds to a maximum at low to intermediate mean current densities and to a minimum when the mean current density is sufficiently high. Also, the higher is the current density, the more marked the minimum. From numerical simulations, it is shown that the current density distribution inversion phenomenon is strongly correlated to the liquid water zone development within the GDL

Liquid Invasion from Multiple Inlet Sources and Optimal Gas Access in a Two-Layer Thin Porous Medium

This study builds upon previous work on single-layer invasion percolation in thin layers to incorporate a second layer with significantly different pore sizes and to study the impact of the resulting water configuration on gas-phase mass transport. We consider a situation where liquid water is injected at the assembly inlet through a series of independent injection points. The challenge is to ensure the transport of the liquid water while maintaining a good diffusive transport within the gas phase. The beneficial impact of the fine layer on the gas diffusion transport is shown. It is further shown that there exists a narrow range of fine layer thicknesses optimizing the gas transport. The results are discussed in relation with the water management issue in polymer electrolyte membrane fuel cells. Additional discussions, of more general interest in the context of thin porous system, are also offered

Characterization of pore network structure in catalyst layers of polymer electrolyte fuel cells

We model and validate the effect of ionomer content and Pt nanoparticles on nanoporous structure of catalyst layers in polymer electrolyte fuel cells. By employing Pore network modeling technique and analytical solutions, we analyze and reproduce experimental N2-adsorption isotherms of carbon, Pt/ carbon and catalyst layers with various ionomer contents. The porous catalyst layer structures comprise of Ketjen Black carbon, Pt and Nafion ionomer. The experimental pore size distributions obtained by N2- adsorption are used as an input to generate porous media using the pore network approach. Subsequently, the simulated porous structures are used to produce simulated N2-adsorption isotherms, which are then compared to the experimentally measured isotherms. The results show a good agreement in the prediction of the effect of the ionomer content on the microstructure of catalyst layers. Moreover, the analysis of the isotherms confirms the hypothesis of ionomer distribution on the surface of agglomerates as well as the existence of different sorption regimes in primary and secondary pores of fuel cell catalyst layers

Performance loss of proton exchange membrane fuel cell due to hydrophobicity loss in gas diffusion layer: Analysis by multiscale approach combining pore network and performance modelling

Loss of hydrophobicity in the gas diffusion layers (GDL) is sometimes suggested as a potential mechanism to explain in part the performance loss of PEMFC. The present study proposes a numerical methodology to analyse this effect by combining pore network modelling (PNM) and performance modelling (PM): the PNM/PM approach. PNM allows simulating the decrease of through-plane gas diffusion coefficient in the GDL as a function of the hydrophobicity loss, which is taken into account through the increase in the fraction of hydrophilic pores in GDL. Then PM based on Darcy equations allows simulating performance loss of PEMFC as a function of gas diffusion decay. This coupling shows that the loss of hydrophobic treatment increases flooding, decreases performance, and increases current density heterogeneities between inlet and outlet of the cell. Interestingly, this degradation is found to be highly non-linear, mainly because of the non-linear influence of the fraction of hydrophilic pores on gas diffusion (this is due to the existence of a percolation threshold associated with the hydrophilic pore sub-network) as well as the non-linear behaviour of electrochemistry with gas diffusion. This study also shows that the loss of hydrophobicity in a GDL is a very suitable candidate to explain performance loss rates that are classically observed during long-term tests. The proposed methodology may also help linking other local properties of components to fuel cell global performance