This paper explores the through-/in-plane characteristics of water transport in the cathode gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC). Theoretical analysis is performed on the non-isothermal two-phase flow under flow channels. A dimensionless group Da (Damkohler number for PEFC operation), defined as the ratio of water generation rate to water vapor-phase removal rate, is formulated to characterize the flow regimes in a PEFC. This group, lumping geometrical parameters and physical properties, compares the water vapor-phase removal capability (via water diffusion and holding capacity) with the rate of water production by the oxygen reduction reaction. We find that this dimensionless group can be used to characterize the non-isothermal, two-phase phenomena: when Da→0, the fuel cell is subjected to single-phase operation; while as Da→ we have full two-phase operation. A more precise expression is explored for the dimensionless group at the channel central line, i.e. Da0: when Da0>1 the entire cathode GDL-CL (catalyst layer) interface is in two-phase region, whereas part of the interface is free of liquid water for Da0<1. The latter scenario is the concept that this paper proposes for improving fuel cell water management: the consequent co-occurrence of single- and two-phase flows in the in-plane direction at Da0<1 is beneficial to avoid severe dryout and flooding. A two-phase transport model, describing the water and heat transport on the PEFC cathode side, is employed to perform a two-dimensional numerical study. Detailed liquid and temperature distributions are displayed. Simulation predictions are in reasonably good agreement with the dimensionless-group analysis. © 2011 Elsevier Ltd.

A multidimensional mathematical model is presented for simulating the coupled phenomena of gaseous fuel/reactant flows, species (including liquid water) transport, heat transfer, hydrogen oxidation, and oxygen reduction reactions in a polymer electrolyte fuel cell (PEFC). The present work focuses on elucidating water distribution in the through-plane direction, in particular across the membrane electrode assembly (MEA) and gas diffusion layer (GDL). Two-dimensional model predictions are computed numerically and compared with available experimental data from neutron radiography or imaging. Using the same set of model parameters, reasonably good agreements are obtained quantitatively between the computed water profile in the MEA-GDL component and the neutron-imaging data reported by two separate research groups, and qualitatively between model prediction and the data from another (third) group. Case-study simulations are carried out for PEFC operation at various temperatures, relative humidities, and current densities. It is found that liquid-water content is lower at higher cell temperatures due to greater water evaporation and stronger water diffusion in the vapor phase, as expected. Without strong water diffusion in the vapor phase, the liquid-water profiles are found to increase with current density in the cathode GDL but indicate a complex trend in the anode. Effect of varying GDL thermal conductivity on water distribution is also examined. © 2010 The Electrochemical Society.

In the present work, a three-dimension (3-D) model of polymer electrolyte fuel cells (PEFCs) is employed to investigate the complex, non-isothermal, two-phase flow in the gas diffusion layer (GDL). Phase change in gas flow channels is explained, and a simplified approach accounting for phase change is incorporated into the fuel cell model. It is found that the liquid water contours in the GDL are similar along flow channels when the channels are subject to two-phase flow. Analysis is performed on a dimensionless parameter Da0 introduced in our previous paper [Y. Wang and K. S. Chen, Chemical Engineering Science 66 (2011) 3557–3567] and the parameter is further evaluated in a realistic fuel cell. We found that the GDL's liquid water (or liquid-free) region is determined by the Da0 number which lumps several parameters, including the thermal conductivity and operating temperature. By adjusting these factors, a liquid-free GDL zone can be created even though the channel stream is two-phase flow. Such a liquid-free zone is adjacent to the two-phase region, benefiting local water management, namely avoiding both severe flooding and dryness.