Microporous Layer MPL Preparation and Optimization

Generally, the hydrophobic property of an MPL is adjusted by adopting different binders and controlling their content. In most cases, PTFE is employed as the binder for a hydrophobic MPL, while in the anodic GDL of a DMFC Nafion is also used in the MPL to make the GDL hydrophilic. A hydrophilic MPL may facilitate methanol transport and improve CO2 removal, resulting in better cell performance [63, 67]. But in most cases, the more important factor influencing 

The pore stracture of the MPL is always tailored by the carbon type and pore former. According to current literature [58, 68], the pores in a GDL are mainly in three size ranges macropores (10-100 pm), mesopores (0.1-10 pm), and micropores ( 0.1 pm). The macropores formed by PTFE are hydrophobic and mainly act as gas passages. The micropores, which are derived from the original 

Bulk porosity, hydrophobic/hydrophilic porosity, and pore size distribution are the most important parameters of the GDL microstructure. Bulk porosity and hydrophobic/hydrophilic porosity are always determined as follows [54, 58]. The sample is immersed in water and then in decane. The water fills only the hydrophilic pores while the decane can fill both hydrophilic and hydrophobic pores. By weighing the sample before and after measurement, total pore volume and hydrophilic pore volume can be calculated. The solid volume of the GDL is calculated using the principle of buoyancy. The bulk or hydrophilic porosity of the GDL is defined as its total pore volume or hydrophilic pore volume divided by the sum of its total pore volume and its solid volume. Hydrophobic porosity is determined by subtracting the hydrophilic porosity from the bulk porosity. Pore distribution is determined by mercury-intrusion porosimetry [54]. Measurement is made of the amount of mercury that penetrates the pores of the sample as a function of the applied pressure. The pressure required for mercury to penetrate a certain size of pore is a function of the pore diameter. The pore size distribution of the GDL is then collected and analysis yields the cumulative pore size distribution. 

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