Water Permeability and Oxygen Transport Resistance of Hydrophilic and Hydrophobic Composite Microporous Layer Coated Gas Diffusion Layer
Authors: Wang Peng , Nakajima Hironori , Kitahara Tatsumi 
Published: December 2024

Water management is vital to enhance the performance of polymer electrolyte fuel cells (PEFCs). Previously, a hydrophilic and hydrophobic composite microporous layer (MPL) coated gas diffusion layer (GDL) was developed to promote robustness to humidity. It was proved that the composite MPL containing hydrophilic and hydrophobic pores in the same layer was effective in reducing the oxygen transport resistance from measured limiting current density under conditions of the cell temperature of 35°C, supplied gas relative humidity of 200% and flow rate of 1000 cm3 min-1. However, directly distinguishing the hydrophobic and hydrophilic microporous structures in the composite MPL is difficult. In this study, we try to clarify the ratio of hydrophobicity and hydrophilicity contributed to liquid water distribution and oxygen transport resistance.
The water permeability test across hydrophilic, hydrophobic, and composite MPLs established the relationship between water flow rates under varying pressure conditions. The water breakthrough pressure obtained with the composite MPL was similar to that obtained with hydrophilic MPL. The composite MPL demonstrated two different rising slopes of the water flow rate. The hydrophilic pores caused the initial slope, which followed the tendency of hydrophilic MPL because water could readily pass through the hydrophilic pores. The following slope changed when the water supply pressure was high enough to pass the hydrophobic pores, and the slope was close to the pure hydrophobic MPL. The water flow rate obtained with the composite MPL was much higher than that obtained with the hydrophobic MPL. Based on the water permeability characteristics, it is considered that the hydrophobic and hydrophilic pores were separately generated in the composite MPL.
To further clarify the contribution of hydrophobicity and hydrophilicity ratio to water transport, liquid water flow rate values for MPLs were derived from Darcy's law to fit the water permeability test data under identical pressure. According to the fitting results, a composition comprising 4% hydrophilicity and 96% hydrophobicity was determined to yield results consistent with the water permeability test, in which the actual composite MPL possessed 10 mass% PVDF, 7.5 mass% Nafion, 2.5 mass% TiO2 and 80 mass% carbon black.
Based on this ratio, the Leverett function was employed to assess the distribution of liquid water saturation within hydrophobic, hydrophilic, and composite MPLs. In the composite MPL, liquid water saturation reached higher levels from the substrate to the hydrophilic pores in the MPL and almost remained constant from the MPL to the MPL/catalyst layer (CL) interface, indicating that the hydrophilic pores were utilized as water transport pathways. Conversely, hydrophobic pores in the MPL were not fully saturated, leaving most pores empty for gas transport. This balanced configuration of hydrophilicity and hydrophobicity substantially reduced the oxygen transport resistance. An additional assessment of the properties ratio was conducted, assuming that the cell operating current density was 3 A cm-2 and the water flow generated in the reaction was obtained. Under the same water flow rate conditions, increasing the hydrophilicity ratio from 0% to 4% resulted in decreased oxygen transport resistance, attributed to a reduction in water breakthrough pressure at the interface between the MPL and CL, facilitated the expelling of liquid water at the MPL/CL interface more effectively compared with a conventional hydrophobic MPL. However, further elevating the hydrophilicity ratio from 4% to 30% resulted in lower water breakthrough pressure but higher water saturation within the MPL. The saturated MPL possessed fewer pathways for gas transport, raising the oxygen transport resistance. Therefore, the inappropriate hydrophilicity and hydrophobicity ratio disrupted the optimal water-gas equilibrium, causing increased oxygen transport resistance. Through a comparative analysis of experimental and calculated data, the optimal formulation emerges, comprising 10 mass% PVDF, 7.5 mass% Nafion, 2.5 mass% TiO2, and 80 mass% carbon black, delineating hydrophilicity and hydrophobicity ratios of 4% and 96%, respectively.