Water-Management Characteristics of the GDL Materials

Cell performance and water management are greatly influenced by the GDL material. Liquid water formation and transport were investigated optically in both cathode and anode channels across the entire cell. We examined the effectiveness of various GDL materials in removing water from the cathode CL into the channels. Level of the cathode flow field flooding was recognized as a criterion for the water removal capacity of the GDL materials. When compared at same current density (i.e. water production rate), higher amount of liquid water in the cathode channel indicated that water had been efficiently removed from the catalyst layer. Our findings show that the cell performance can be increased several times solely by manipulating the water management properties of the GDL material.

Figure 1 - SEM images of GDL media

Figure 2 - Cathode channel flooding with different GDL materials

Anode Flooding and the Role of the Microporous Layer (MPL)

Special attention is devoted to the role of the microporous layer (MPL), due to its conflicting effects on water transport and cell durability. For improved water management, GDLs often have a dual-layer structure; with an MPL, highly hydrophobic coating with fine pore structure, deposited on a carbon fiber substrate (such as the one shown in Fig.1c). The MPL is less sensitive to flooding due to higher saturation pressure in the small hydrophobic pores. Our studies indicate that the GDL acts as a pressure valve for water condensed at the CL/GDL interface. The burst pressure of this ‘valve’ can be adjusted by tailoring the wetting properties of the GDL, and is increased five times when the MPL is present. Due to the high pressure barrier at the cathode CL/MPL interface, water is pushed across the membrane to the anode side. This effect was evidenced by optical imaging of liquid water accumulation in the anode channels. This is a unique study since the water distribution in the anode flow field was for the first time directly visualized.

Water on the anode side of the cell may cause the same negative effects as on the cathode side, by hindering the gas access to the CL. Besides the instantaneous drop in the performance, even more important is the irreversible damage to the cathode CL, via accelerated carbon corrosion caused by localized fuel starvation. Our experiments reveal that the anode channels do not exhibit liquid water accumulation unless the cathode GDL has an MPL. This poses an optimization problem regarding the use of the MPL, as the benefits of improved cathode water management and membrane hydration may be negated by the long-term damage caused by the anode flooding.

Spernjak D., Prasad A.K., and Advani S.G., "Experimental investigation of liquid water formation and transport in a transparent single-serpentine PEM fuel cell," Journal of Power Sources, Vol. 170, pp. 334-344, August 2007. doi:10.1016/j.jpowsour.2007.04.020