Development of a porosity-graded micro porous layer using thermal expandable graphite for proton exchange membrane fuel cells

In this study, a porosity-graded micro porous layer (MPL) was prepared using the double coating method to enhance the water removal ability of the gas diffusion layer (GDL). In the double MPL, the porosity of each layer was controlled using thermal expandable graphite (TEG), which could produce pores in MPL through thermal expansibility. The porosity of the inner layer of the porosity-graded MPL was smaller than the outer layer, so the gradient direction in porosity was from the MPL/catalyst layer interface to the gas diffusion backing layer (GDBL)/MPL interface. In addition, the pore forming ability of TEG and the water removal ability of porosity-graded MPL were characterized. The performance of the porosity-graded MPL was evaluated and compared to the single layer conventional MPL. The porosity-gradient structure in MPL increased the water permeability of GDL and the performance of the single cell in the high current density region. Since the porosity-graded MPL increased the water removal ability of GDL, concentration loss due to water flooding in the high current density region was decreased. These results demonstrate that porosity-graded MPL was beneficial to PEMFC, which has to operate in the high current density region.