The Change of Water Distribution in PEMFC according to the Pore Gradient in GDL : The Effect of the Water Accumulation at the Interface between GDL and Gas Supplying Channel

Abstract

This research quantifies how the pore gradient in the gas diffusion layer (GDL) affects the electricity generation efficiency of polymer electrolyte membrane fuel cell (PEMFC) by affecting the water distribution in it. Analysis considered three GDLs: one with no pore gradient, moderate gradient and high gradient. A test of performance and X-ray visualization were conducted on the three GDLs at two relative humidity conditions to determine how the structure of GDL affects the water distribution at the different water supplying condition. The result showed the low water saturation in PEMFC with GDL having pore gradient. This change decreased the concentration over-potential and increased performance at high RH condition. However, the GDL having high pore gradient cannot show the improved performance at low RH condition. The extra water removal according to steep pore gradient generated membrane dehydration problem and increased Ohmic over-potential. This result means that the GDL with the highest pore gradient cannot guarantee the highest performance. Next, the change of water distribution according to the porous structure of GDL was analyzed based on the water accumulation at the interface between GDL and gas supplying channel. The amount of water at the interface accumulated until it detached from the GDL was visualized by X-ray. The result presented that higher the water saturation in PEMFC is matched with the larger size of water detachment. The different size in water detachment was caused by the different adhesion forces in the 3 GDLs, so the adhesion force was measured by tilting the surface until a water droplet on the GDL surface detached from the surface. The adhesion force was calculated from the gravity force and tilting angle. The result was compared to the predicted value by precedent models. However, precedent models cannot predict the adhesion force precisely due to the insufficient consideration of triple line. Therefore, in this research, the shape of the deformed droplet was modeled and the adhesion force was suggested based on it. The proposed model predicted the adhesion force precisely, while the precedent models predicted the adhesion force more than 30% error. The importance of the precise prediction of adhesion was investigated using simple numerical simulation. The boundary condition of water saturation at the interface between GDL and gas supplying channel was set to be proportional to the adhesion force and its effect on the water distribution was analyzed. The result showed that the boundary condition strongly affected the water distribution in PEMFC. This result means that the water distribution in PEMFC requires the quantification of the adhesion force.