Sang-Yeop Lee

Effects of Purging on the Degradation of PEMFCs Operating with Repetitive On/Off Cycles

Degradation of polymer electrolyte membrane fuel cells (PEMFCs) that is facilitated by on/off cycles is one of the most important issues for commercialization of fuel cell vehicles. When a PEMFC stack is shut down, residual hydrogen could induce high voltage equivalent to open-circuit voltage to the cathode side that might cause sintering of the Pt catalyst and facilitate the formation of hydrogen peroxide radicals at the anode side that might decompose the Nafion membrane. In this study, the degradation of PEMFCs exposed to repetitive on/off cycles was investigated by measuring current density-voltage characteristics, ac impedance, cyclic voltammograms, gas leak, cross-sectional scanning electron microscopy images, and transmission electron microscopy images. To prevent the degradation of PEMFCs caused by the residual gases, hydrogen was removed from the anode gas channel by air-purging, which was found to be a very effective method.

Complex Capacitance Analysis of Ionic Resistance and Interfacial Capacitance in PEMFC and DMFC Catalyst Layers

For polymer electrolyte membrane fuel cell (PEMFC) and direct methanol fuel cell (DMFC) catalyst layers (CLs), a complex capacitance analysis of impedance data was developed to evaluate the catalyst/ionomer interfacial capacitance and ionic resistance of ionomer networks without nonlinear data fitting. First, assuming no faradaic reactions, equivalent circuits for the CLs were suggested, which are similar to electric double-layer capacitor systems with porous carbon electrodes. Then, with the simulated complex capacitances, it was confirmed that the plots of the real and imaginary parts as a function of ac frequency are determined by the catalyst/ionomer interfacial capacitances and the ionic resistances time constants, which are important characteristics for high fuel cell performances. Experimentally, the condition of no faradaic reactions was realized by supplying nitrogen or water to the cathodes instead of air and by fixing the dc potential at 0.4 V during the impedance measurements. By performing a complex capacitance analysis, the interfacial capacitances and ionic resistances of PEMFC membrane electrode assemblies (MEAs) can be obtained at various relative humidities, proving that the catalytic activity in fuel cell operation depends on ionic resistances as well as on the catalyst/ionomer interfacial area. The effects of various MEA preparation methods on the ionomer distributions and DMFC performances were analyzed by a complex capacitance analysis.

Effects of Shut-down Process on Degradation of Polymer Electrolyte Membrane Fuel Cells I. Effects of Hydrogen Removal on the Degradation

Degradation of polymer electrolyte membrane fuel cell (PEMFC) that is facilitated by on/off cycles is one of the most important issues for commercialization of fuel cell vehicles. When a PEMFC stack is shut down, residual hydrogen and induce high voltage equivalent to open circuit voltage to the cathode side that might cause sintering of Pt catalyst and facilitate formation of hydrogen peroxide at the anode side that might decompose membrane. In this study, degradation of PEMFC exposed to repetitive on/off cycles was investigated by measuring i-V characteristics, ac impedance, cyclic voltammograms, gas leak, cross-sectional SEM images, and TEM images. To prevent degradation of PEMFC caused by the residual gases, hydrogen was removed from anode gas channel by gas-purging and by using a dummy resistance, that were found to be a very effective method.