Water Management in A PEMFC: Water Transport Mechanism and Material Degradation in Gas Diffusion Layers

Abstract

It has now been well recognized that both the performance and durability of proton exchange membrane fuel cells (PEMFCs) are closely related to the water accumulation and transport inside its porous components, particularly in the gas diffusion layer (GDL), and microporous layer (MPL). In this paper, the key GDL and MPL properties that affect water transport through them are first discussed and a review of GDL degradation mechanisms is presented. An intermittent water drainage mechanism across the GDL is discussed. The capillary breakthrough pressure (CBP) and the dynamic capillary pressure (DCP), or recurrent breakthrough dynamics, have been identified as key GDL properties that affect its water management performance and function as indicators of the degradation of GDL material. This work uses a novel ex situ experiment to degrade a GDL by exposing it to an accelerated stress test (AST) that subjects the GDL to elevated operation conditions seen at the cathode side of a PEMFC for an extended period of time. In turn, the effect of the AST on the CBP and DCP is investigated. As a result, a loss of hydrophobicity occurred on the MPL surface. This altered the CBP and DCP, thus decreasing water management in the GDL.

Degradation in PEM Fuel Cells and Mitigation Strategies Using System Design and Control

“…Degradation of the GDL can reduce or block gas passageways resulting in choking of the fuel cell. A major source of GDL degradation has been due to mechanical compressions resulting in stress and strains that reduce the micro-pore regions [24]. This reduces gas transport and thus reduces the effectiveness of the GDL.…”

“…Freeze thaw cycles are well known to damage the GDL as freezing water expands and damages the micro-pores. Another source of damage to the GDL is the loss of polytetrafluoroethylene (PTFE) [5,24]. PTFE is used to maintain gas passages by repelling water using its hydrophobic properties.…”

“…Corrosion of the carbon layers due to oxidation and due to dissolution of the platinum catalyst can slowly erode the PTFE layers. For long duration fuel operations, the major degradation mechanism ends up being GDL degradation particularly the gradual loss of the PTFE [5,24].…”

“…The erosion of the PTFE results in accumulated flooding in the fuel cell [24]. Flooding is known to cause corrosion of various components of the fuel cell including the catalyst [25].…”

Modification of gas diffusion layers properties to improve water management

“…Both parts are treated with poly(tetrafluoroethylene) (PTFE) to obtain specific hydrophobicity needed for fuel cell operation. Flooding of the active catalyst layer (CL), where the reactions take place [4] is avoided by transporting the water away from the reaction layer due to the hydrophobic behavior of GDL [5,6]. Water produced inside the fuel cell can flood the pores of GDL and decrease the gas permeability of the layer promoting fuel starvation [7][8][9].…”