Analysis of Water Transport inside Hydrophilic Carbon Fiber Micro-Porous Layers with High-Performance Operation in PEFC

Abstract

Polymer electrolyte membrane fuel cells using a hydrophilic micro-porous layer (MPL) consisting of carbon fiber (CF) and ionomer show better performance than those using conventional hydrophobic MPL with carbon black (CB) under a wide range of humidity conditions. This study investigates the effects of the wettability and structure of the MPL on cell performance and discusses the mechanism for improving cell performance by the hydrophilic CF-MPL at high humidity conditions. The water distribution inside the MPL is evaluated by a freezing method and cryo-SEM observations, and a comparison of the distribution and the cell performance of hydrophilic and hydrophobic MPL cells with various structures is made. The results show that the performance improvement is caused by preventing increases in concentration overvoltage at high current densities; where the fiber structure and thicker layers are important with hydrophilic MPL. In hydrophilic CF-MPL with better performance, liquid water accumulates in pores at the catalyst layer (CL) side, while there is no liquid water at the gas diffusion layer side. These results indicate that the hydrophilic CF-MPL enhances the liquid water transport from the CL due to the hydrophilic properties, and promotes water evaporation due to the large pores in the thick fiber structure.

Enhanced Water Management of Polymer Electrolyte Fuel Cells with Additive-Containing Microporous Layers

“…In recent years, many researchers have explored the effects of the MPL wettability and microstructure on water management, and novel MPL design concepts have emerged. − Doping the MPL with various nanomaterials has proved to be an effective way to improve water management and increase potentials at high-current-density operation, although the exact mechanisms for water removal and the concomitant increased performance are not well understood. − Many of these dopants are hydrophilic in nature, which questions the convention of having a hydrophobic MPL interfaced with the cathode. In fact, Weber predicted in a modeling study that the addition of hydrophilic pathways through an MPL could help increase performance by providing hydraulic conductivity between the catalyst layer and GDL, thus enabling more-efficient water removal .…”

Highly Ordered Nanoporous Carbon Scaffold with Controllable Wettability as the Microporous Layer for Fuel Cells

“…In one study, it has been proposed that the hydrophilic MPL provides a higher surface area for water to distribute, thereby enhancing the capacity of the CL to retain water without flooding . A counter proposition has been made that the hydrophilic pathways alleviate flooding by selectively removing liquid water from the CL, in concurrence with predictions from a modeling study .…”

“…Modification of the MPL microstructure by the addition of carbon nanotubes to the typical carbon black/Teflon mixture for MPLs , or by the replacement of carbon black with graphene has been found to improve fuel cell performance . A number of recent studies have also reported MPLs prepared from the hydrophilic additives, such as hygroscopic polymers, − hydrophilic CNTs, − and inorganic oxides. ,, In certain studies, it has been proposed that fuel cells with a hydrophilic MPL when operated under dry conditions have shown better performance in the so-called ohmic loss-dominated region of the cell polarization behavior, owing to the higher water retention in the membrane and CL but exhibited higher mass transport losses at high humidities , In contrast, other studies have reported that fuel cells with a hydrophilic MPL have performed significantly better in the mass-transport loss-dominated region regardless of the hydration level. ,, …”

“…53,56,57 In certain studies, it has been proposed that fuel cells with a hydrophilic MPL when operated under dry conditions have shown better performance in the so-called ohmic loss-dominated region of the cell polarization behavior, owing to the higher water retention in the membrane and CL but exhibited higher mass transport losses at high humidities 49,50 In contrast, other studies have reported that fuel cells with a hydrophilic MPL have performed significantly better in the mass-transport loss-dominated region regardless of the hydration level. 52,53,57 In one study, it has been proposed that the hydrophilic MPL provides a higher surface area for water to distribute, thereby enhancing the capacity of the CL to retain water without flooding. 52 A counter proposition has been made that the hydrophilic pathways alleviate flooding by selectively removing liquid water from the CL, 53 in concurrence with predictions from a modeling study.…”

Influence of Structural Modification of Micro‐Porous Layer and Catalyst Layer on Performance and Water Management of PEM Fuel Cells: Hydrophobicity and Porosity

“…The small pores in MPLs containing a hydrophobic binder cause high capillary pressure, as a result; the liquid water is both withheld in the CL and preferably transported through cracks and larger pores [13]. Numerous studies have investigated the effects of microstructure properties, especially the wettability [14][15][16][17][18][19][20] and porosity of MPLs [21][22][23] on water balance and performance of PEMFC. Theoretical models have predicted that the introduction of adjusted macro-pores in the catalyst layer leads to better performance.…”