Experimental and theoretical study of cathode catalyst layer in PEM fuel cells
Date of Issue2011
School of Mechanical and Aerospace Engineering
Proper water management is the key to achieve performance stability in a polymer electrolyte membrane fuel cell (PEMFC). Effects of the properties of the cathode catalyst layer (CCL), where oxygen reduction reactions (ORR) and water generation occur, on water management and the performance of the fuel cell have been investigated thoroughly. To fulfil the objectives of this PhD study, a novel anti-flooding CCL has been developed with the addition of hydrophobic but oxygen permeable dimethyl silicone oil (DSO) into the CCL to achieve the water balance and improve oxygen transport within the cathode. Several characterization techniques were employed to understand the relationship between the cell performance and CCL properties. The results indicate that the loading of DSO in CCL plays a critical role in preventing the cathode from flooding and facilitating the oxygen transport under over-saturated conditions at both room temperature and elevated temperatures. An optimal DSO loading in the CCL was found to be around 0.5 mg/cm2 under the testing conditions in this work. The role of micro-porous layer (MPL) was found to be critical for the efficient water management in the PEM fuel cell. Since the pore size in the CCL is in the same order of magnitude as that in the MPL, the hydrophobic CCL modulated by DSO can function like a watershed as the MPL. The fuel cell with DSO loaded CCL without MPL has slightly poorer performance than that with MPL in a normal CCL, at low current density, but it outperforms the latter at high current density. The cathode with both MPL and DSO loaded CCL is performed well in highly humidified condition, but it suffers from high ohmic loss at low current density region due to the poor water retention ability of the cathode. Hence, the hydrophobic level of the CCL is critical and must be carefully controlled to balance the conflicting requirements of the electrolyte hydration and the cathode anti-flooding. A mathematical model, describing the major transport phenomena in cathode catalyst layer (CCL) of a PEMFC, has seen developed. It reveals how the hydrophobicity of the CCL surface steers water, oxygen concentration and current density distributions in the CCL, thus affecting the performance of a PEMFC. The roles of wetting properties of the CCL on controlling the water transport under some operating conditions were simulated. This one-dimensional CCL model can help to fundamentally understand the water transport within the CCL and optimize the operating conditions and the CCL properties for improved fuel cell performance and its stability. In the light of the results achieved from the systematically experimental and modelling study, it is fundamentally clear that the optimized hydrophobicity of cathode catalyst layer is effective in expelling excessive water from the cathode, thus preventing the flooding and improving the fuel cell performance.