Two-Phase Transport in Polymer Electrolyte Fuel Cells with Bilayer Cathode Gas Diffusion Media

Abstract

A two-phase, full cell model based on the multiphase mixture ͑M 2 ͒ framework is developed to analyze the two-phase transport in polymer electrolyte fuel cells with bilayer cathode gas diffusion media ͑GDM͒, consisting of a coarse gas diffusion layer ͑GDL͒ with an average pore size around 10-30 m and a microporous layer ͑MPL͒ with an average pore size ranging from 0.1 to 1 m. Effects of the relevant properties of the MPL on liquid water transport are examined, including average pore size, wettability, thickness, and porosity. It is quantitatively shown that the MPL increases the rate of water back-flow across the membrane toward the anode by increasing the hydraulic pressure differential across the membrane, consequently reducing the net amount of water to be removed from the cathode. Furthermore, it is seen that different microporous and wetting characteristics of the MPL cause a discontinuity in the liquid saturation profile at the MPL-GDL interface, which in turn reduces the amount of liquid water in the catalyst layer-MPL interface. Our analyses show that the back-flow of liquid water increases with increasing hydrophobicity and thickness, and decreasing pore size and porosity of the MPL.A main limitation in polymer electrolyte fuel cell ͑PEFC͒ performance results from the transport of reactants from the channel to the catalyst layer, referred to as the mass-transport limitation. This limitation is further amplified by the presence of liquid water, which blocks some of the open pores in the gas diffusion media ͑GDM͒ and thus reduces the available paths for the transport of reactant species. This phenomenon, commonly referred to as flooding, is more severe in the cathode because the slower oxygen reduction reaction ͑ORR͒ is more susceptible to the negative impact caused by flooding. Recently, a bilayer GDM, consisting of a coarse gas diffusion layer ͑GDL͒ and a finer microporous layer ͑MPL͒, has been employed by practitioners to reduce flooding in the porous cathode and to enhance water management in PEFCs by increasing the backflow tendency of liquid water across the membrane toward the anode. It has been shown that highly hydrophobic MPLs usually exhibit better performance. 1-13 Although the exact mechanisms are yet to be fully elucidated, the performance enhancement is usually associated with better water management capabilities of MPLs. In phosphoric acid fuel cells, Hara et al. 2 wetproofed the GDL using a fluorinated polyethylene film, which has much smaller pore size than commonly used polytetrafluoroethylene ͑PTFE͒ particles. It was found that this additional layer, which is similar to the MPL in PEFCs, improved oxygen reduction by reducing the flooding. Passalacqua et al. 3 also showed that introducing a hydrophobic layer between the carbon paper and the catalyst layer improves cell performance. The thin hydrophobic layer of about 2 mg/cm 2 of carbon ͑Vulcan XC-72͒, containing 40% of PTFE, substantially improved cell performance both in air and pure oxygen operation, by reducing the ohmic losses and i...

Impact of micro-porous layer on liquid water distribution at the catalyst layer interface and cell performance in a polymer electrolyte membrane fuel cell

“…For practical uses of PEFC, water flooding at high current density conditions is one of the major issues to be improved, because flooding deteriorates efficiency and maximum power output due to limiting the supply of reactants to the reaction area by the water accumulated in the cell. It is generally known that a micro-porous layer (MPL) contributes to better water removal from the cathode catalyst layer (CL) and to improvements in the cell performance [1][2][3][4]. The MPL introduces a -2 -fine carbon layer between the CL and gas diffusion layer (GDL), and has smaller pore sizes than the GDL.…”

“…Early computational studies reported that the MPL improves water removal from the cathode GDL by increasing the hydraulic pressure differential across the membrane [1], and that the MPL acts like a valve that pushes water away from the cathode and towards the anode through the membrane [2]. Gostick et al estimated the water saturation and capillary pressure for GDLs with and without an MPL from the measured conditions of the liquid water breakthrough at the porous layers, and suggested that the efficacy of the MPL is due to finite-size effects related to invasion percolation in thin GDLs where liquid water percolation through the MPL results in limited access of water to the GDL inlet face [5].…”

In situ accelerated degradation of gas diffusion layer in proton exchange membrane fuel cellPart I: Effect of elevated temperature and flow rate

“…Past studies have shown that the properties of both the GDL substrate and MPL play a significant role in water balance and performance of proton exchange membrane (PEM) fuel cells [1][2][3][4][5][6]. In these studies of GDLs, the impact of GDL materials and design on PEM fuel cell performance losses, rather than durability, has been the focal point.…”

Modelling water intrusion and oxygen diffusion in a reconstructed microporous layer of PEM fuel cells

“…The earlier theoretical 4 models hypothesized that the water produced in the cathode condensed in the CL first, and it was then driven into the MPL by a capillary pressure gradient [23]. Since the MPL was assumed to be more hydrophobic than the CL, these models predicted that with an increase in MPL thickness, the water would gradually flood the part of MPL adjacent the CL.…”