LASER PERFORATION AND IMPROVEMENTS OF MASS TRANSPORT OF DIRECT METHANOL FUEL CELLS

Abstract

Mass transport in gas diffusion layers plays a pivotal role in a direct methanol fuel cell (DMFC). Gas diffusions layers, which are located in between catalyst layers and flow channels, are responsible for delivering reactants and removing products to/from the electrochemical reaction sites in the catalyst layers. At the anode, the removal of CO2 is a major issue, and methanol transport to the anode catalyst layer needs to be enhanced without significantly increasing methanol cross-over from the anode to the cathode that hinders the cathode catalyst utilization. In this work, novel anode gas diffusion layers (AGDLs) with both hydrophobic and hydrophilic pathways are created to enhance transfer of both methanol and CO2 at the anode compartment. Such AGDLs are created by perforating PTFE-treated GDLs with laser, so that the original pores/pathways in the AGDL are hydrophobic and the laser perforations are hydrophilic, thus providing easy transport paths for both the liquid methanol solution and CO2 at anode. The performances of fuel cells with some of the novel AGDLs are significantly higher than the cell with unperforated GDL. The perforated AGDL with 60µm diameter and moderate perforation density has shown the best performance with a maximum peak power density of 89 mW cm-2 , an increase of about 32% compared to the non-perforated AGDL. Results of electrochemical impedance spectroscopy (EIS) show that the main reason for the performance enhancement is due to the reduction in charge and mass transfer resistances. The charge transfer resistances are reduced due to the enhanced methanol transfer to the catalyst layer. The results of linear sweep voltammetry (LSV) show that the perforations can in fact increase methanol crossover, thus if perforation density of the AGDL is too high, the cell performances are lower than that of the virgin AGDL. Similar to the anode mass transport issue, the cathode also suffers from the issue of the water removal and oxygen transport that are in opposite directions. In this work, novel cathode gas diffusion layers (CGDLs) with hydrophilic perforation by laser in a generally hydrophobic CGDL are used to enhance oxygen transport and water removal at the cathode compartment. The perforations create pathways for oxygen to accelerate the oxidation of the cross-over methanol and free more reaction sites for the oxygen reduction reactions. The hydrophilic perforations within the GDLs are used to enhance the ejection of excessive water to the channels while the rest of the GDLs that is hydrophobic can help to balance water in the cathode catalyst layer. Furthermore, a perforated CGDL with 100µm diameter and high perforation density has shown higher performance than the unperforated GDL by 16% in maximum power density. EIS results for low and high current density regions indicate that the use of perforations is advantageous for the high voltage region but could be disadvantageous in the low voltage region, especially when larger perforation diameters are used. Ohmic, charge and mass transport resistances are studied and analyzed by EIS to further explain the variations in performance shown in polarization curves. Besides, when lower methanol concentration is used, cathode GDL perforation showed highest performance for the largest perforation diameter.